I want just to make sure I understand the boss gear exchange coupon for Tuvala correctly:

You have to graduate and convert the Tuvala gear from "season gear" to "regular gear" first, right?

So, that means, once you have a full set of PEN Tuvala there is no point in enhancing any more Tuvala items?

(and you should probably exchange the time filled black stones for Advice of Valks instead)

Why I'm asking this:

If you could exchange Tuvala for boss gear before graduation, you could create another piece of PEN Tuvala for the exchanged items... (which would be cool)

When you graduate, you have the opportunity to convert a full set of seasonal Tuvala gear to regular Tuvala gear. This then makes it so that a regular character can use them instead of a seasonal character but doesn't otherwise change the gear's stats or what you can do with it, and it's impossible to ever enhance regular Tuvala gear, so it's stuck at whatever level it was at when the conversion happened. You then also get a single coupon to convert a piece of regular Tuvala gear to the equivalent boss gear (e.g. a PEN Tuvala main-hand weapon becomes a TET Kzarka weapon).

As for making more than a full set of PEN Tuvala seasonal gear, no, there's no point. Any extras disappear when the season-specific stuff gets removed after the season. That being said, you can use Tuvala gear to build failstacks to 80 relatively easily, and because you can get Time-Filled Black Stones more easily than regular Black Stones, it's a good source of failstacks. You can also just convert Time-Filled Black Stones to Advice of Valks directly, but they're obviously not as large that way. There's a member of my guild who does seasons specifically for the failstacks and doesn't care that much about the rest, whereas some people just want to get their seasonal character turned into a regular one as quickly as they can and forget that the season was even a thing until the next season. Of course, they then also lose out on the opportunity to do stuff like grind for potion pieces with fewer players around, but some players are just after that PEN Capotia item and/or the single piece of TET boss gear.

Thanks for the answers!

I have not been too much into enhancing so far, so this definitely is an interesting perspective. From that perspective it would probably have been smarter to concentrate on building fail stacks and to consider getting PEN Tuvala gear a useful byproduct of that process (instead of the other way round).

I will think about what I will actually do until the season ends... (still have time :) )

Also a friendly PSA from a hard lesson learned: The Tuvala Weapon Exchange coupons you get from using the Time Piece or from turning in 40 Swell Sea Shells cannot be used on prior Seasons' Tuvala weapons, you can only use them on current Season Tuvala Weapons. IMHO it makes no sense to have 2 sets be given out when after you use the coupons from the Time Piece it renders the Swell Sea Shell turnins redundant and worthless.