do you need to complete the main quests (from the start areas til ulukita) on every character again, to unlock all content for the characters? or is it enough to complete the main quests just only one character? or can you skip the main quests somehow, like on the season server?

No idea what you mean by content, but you need to compelete story quests at least once to unlock some family based stuff behind main story quests. So technically you need to finish mainstory once.

with content i mean everything, where we have to complete the main quests first. in other words: If you don't complete the main quests, does that prevent you somehow from being able to use things / having access to something on the characters?

no

You don't *have* to do anything, even on your first character.  It's up to you if you want to do quests at all.

What you get from doing quests:
- Knowledge (and associated energy)

- Contribution points

- Skill and combat XP (mostly for side quests)

- Inventory space for some of the starting main quests

- Quest count for Rulupee's log (for titles)

- Some quest gear that's useful to have when leveling alts.

If none of that interests you, you don't have to do quests at all. You can hide them all in your menu and do whatever else you feel like instead.

No, but I usually do them for quest count and to make a rebellious crystal at the end of the Kama one

Good to do them for quest count for journals, just be aware that deleting a character also deletes the quests completed from your adventure journal.