Before I begin, this is my personal experience about the class as a main Sage, thus my personal opinions about it;

I've played the class since release, the mechanics, lore and character's model just made fall in love with the class overall.

I play both succession and awakening stances actively as their performance changes within contexts and each performs a little bit better than the other in certain circumstances.

There have been many updates along with huge class reworks which left Sage and other classes which haven't been paid enough attention at on the bench.
Sage's stats have remained the same of a META which no longer exists and it should be addressed as soon as possible.
The class can no longer compete fairly with any of the other classes, due to its lack of protections, damage and other modifiers which just make it hard if not impossible to fight against someone else, moreover, the PvE experience isn't as good as it used to be either.
It's hard to trade between protections as the class lacks of them, sometimes is too slow, consumes too much MP and Stamina and just is too squishy.
Here, I'll do my best to suggest changes and state a reason as to why these changes could get applied;

■ Succession
● Prime: Void Gateways
- Add Frontal Guard or Super Armor.
- Increase Casting speed.
- Make it cancellable.
Void gateways happens to be one of the most useful and strongest abilities when it comes to succession, especially for PvE, however, it's hard to make it really  useful when it comes to it.
On the PvP aspect, the ability has too long of a casting time and it's hard to land that floating or air smash on it, especially when experiencing desync.
The ability is also unprotected and not cancellable, which means that most of the time you'll die while being stuck in it or get CC'd down.
On the PvE aspect, it has fair modifiers, however, if you don't have enough resistance to mobs's CCs or a decent amount of evasion, there is a good chance of being CC'd and the ability will just go on cooldown without being casted.

● Prime: Ator's Mark
- Change Frontal Guard to be applied during the whole ability's animation, not just while casting.
- Allow us to move the camera to reposition the ability's location when the casting animation is cancelled.

After using an ability and then casting Ator's mark, the ability will just land at the same location of the previous one, without letting us move around to fire it at the location angle we're looking at.
This leaves Sage off-guard without the chance of hitting a target that isn't CC'd and is bolting behind us; allow us to move more freely around ourselves.
Adding frontal guard during the whole animation would help compensate the gap between the end of the casting time and the end of the ability itself, which really isn't too short.

● Prime: Form Shift

- Add once again the critical rate.

● Prime: Spatial Collapse

- Change Frontal Guard to be applied during the whole ability's animation, not just while casting.
- Reduce the cooldown from 11 seconds to 10 seconds.

● Flow: Finishing Touch

- Reduce the cooldown from 17 seconds to 15 seconds, or change it to match Spatial Collapse's one (as it used to be in the beginning).

● Prime: Ator's Energy

- Increase Accuracy rates from +7% to +10%.

● Prime: Atomagia

- Reduce the cooldown from 40 seconds to 20~15 seconds.
Cutting down the cooldown on said ability would be a huge difference, allowing us to trade Super Armor easier and allowing us to actually use the ability more often.
I understand the high cooldown might be due to the high damage and critical modifier, but I don't think it's worth it.

● Prime: Spear Bolt
- Increase Damage on both attacks.
- Increase Magic AP gain when using the skill from +10 to +15.
- Add Stiffness on spear's attack (?).
- Possibly increase attack range.

- Decrease Cooldown from 15 seconds to 12 seconds.
Here, I too understand the low damage due to the fact that it's a teleport ability which dashes to the target when succesfully hit and when in short range, but the ability gains no other purpose out of that due to low modifiers.
As one of the two unique succession abilities made to resemble the awakening's kit, it'd be good to actually do that.
Other than that, I believe that cutting down the cooldown and increasing the AP gain would indeed resemble more awakening's spear bolt and it'd probably come more handy in the kit.

● Prime: Rift Storm
- Cut down the Cooldown from 13 seconds to 11 seconds.

● Prime: Rift Chain

- Add an Evasion buff when on Cooldown.
- Possibily remove character's collision on Cooldown.
- Look into iframe not activating properly at the beginning of the ability.

- Reduce the Stamina consuption.
As stated in another post, no one wants a return of permanent iframe, neither in awakening or succession.

However, when it comes to succession and rift chain being on cooldown, the DP buff and the speed at which sage gets in and out of the ability isn't fast enough.
Therefore, most of the times it's a sentence to death.
Adding an evasion buff when using the ability would possibly help in certain cirumstances, since Sage has a good base of evasion %, it may be noticeable.
Reducing the stamina consuption and increasing the speed of the get in-out animation of the ability would represent a good change already.

● Prime: Optimization

- Add a +20% Casting Speed for the duration of the ability (30 seconds).

● Prime: Ancients' Wisdom
- Add a Casting Speed modifier equal to +0.5%~+1% at each level.

My biggest concern is Sage's casting speed, compared to other magic type classes, I find him to be one of the slowest if not the actually slowest one.
It sure made progress since the beginning when it felt like we were on a constant inventory's overweight.
I know we do have a passive which can grant us up to +10% casting speed when at max stack, but that just isn't enough, and with the new reset mechanic the buff is completely inexistent.
Adding a balanced amount of extra casting speed would be a great improvement for the whole class' performance.
Note: I use a Glorious Crystal of Gallantry and Vell's Heart which give me an extra +6% of casting speed and I can notice a little difference.

● Reset

- Reduce the cooldown.
I love the idea and mechanic of Sage being able to reduce and reset his cooldowns but given the latest update on the mechanic, Reset has become a burden to use, due to the fact that it consumes all 10 stacks of Overdrive for such a long duration.
Shortening the cooldown could help balancing out the casting speed trade.

■ Awakening 
Here I'll begin by saying that the biggest issue of the awakening kit is the absurd stamina consuption for a single mobility chain, sage has no way of defending himself without stamina and will just be stuck when drained.
That really needs to change.

● Bolt
- Reduce the Stamina consuption.
- Give iframe when off Cooldown.
Here too, we've been through perma iframe era and we don't want that, but I don't see why we wouldn't get it when the ability is off cooldown just like other "normal" classes.
Awakening sure is harder to track than it is in succession but I wouldn't consider giving an iframe too big of an issue for opponents.

● Divine Executioner

- Issue with iframe not applying correctly when invisibile (might be me experiencing desync but it's worth noting).
- When teleporting to target, Sage will sometimes glitch into obstancles in the way.

● Impaling Flash

- When teleporting, Sage will glitch through or underneath objects, causing him to get stuck or die from fall damage.

● Lightning Prison

- Give back Critical Hit rates (at least +30%)
- Increase the HP recovery from 25 to 30 per hit.

● Ator's Spear

- Increase Accuracy Rate from +5% to +7%~+8%

Overall, damage modifiers should really get looked into, and the latest damage nerfs should be reverted.
I've been more focused about succession as I believe that @SorcSupreme explained and stated very well each of their points here, and might just be what Sage needs right now.
More attention should be put onto Sage since it's a class that has been released and forgotten about, over the past few months and since release really, we've seen nothing but nerfs which led the class to a stagnant state of weakness, without any chances to compete with other classes.
Please keep in mind that this is just my experience and thoughts on the current class' performance.

Edit:
● Prime: Ator's Fist

- Add extra hit.
The ability could be improved to have an extra hit, from which a second fist comes out by using sage's other arm.

There are new buffs for Awakening Sage in the latest global lab patch note (May 20th) + some functionnal changes on Impaling Flash

https://www.blackdesertfoundry.com/global-lab-updates-20th-may-2022/#Sage

Succ Sage was good when released than people started to cry because they could not mindless use their i win grab button on him so they took away the only good thing he had: his Iframes.. but why? he has no grab he has the slowest animations ever and sorc can also spam iframes as she likes WHY THE HELL u took it away?!

because everyone and their mother rerolled to it back then and were making montages that even made todays succ zerkers 200% clips feel inadequate? 

The only problem they should have fixed was, ebuff, go in, kill, go out and autoattack till ebuff is up. then repeat.

thats the reason why the death/kill ratio was so good. Sage overall was fine. just end Ebuff spam