Create an item that decrease the xp recieved

[Jeeqbit]

This either means losing a lot of xp, or spending a lot of time leveling another job before the msq can take over.

Slowing the leveling and throwing away the xp is a bad idea to me though, as i think that this xp should at least be granted back through another route.

This doesn't matter because experience is easy to get in this game. For example, you can go and do beast tribe quests and get a massive chunk of exp each day for 10 minutes of work (or less). Additionally, you are rewarded for this if you didn't fully rank the tribe yet.

And overleveling has a serious negative effect on some fights. Especialy when you have those open world encounters, these dont scale up. So a lvl 83 enemy just gets 1 shotted

Your level increases, but your gear does not. If you level without increasing your gear, you actually become weaker, because level dilutes stats. The only thing that makes up for this at all is new actions, abilities and potency increases, so it evens out enough.

Most fights take place in an instance, and due to this, the level gets synced anywhere. Therefore, it's not a problem in my opinion.

Also, in each expansion i was able to level 3 jobs at once easily

I can't relate to that because, for me personally, the exp is just enough. People who advise sprouts to level multiple jobs this way trap them when they have to find a way to level up the other job to the level of the MSQ and waste their time when they could be enjoying the story.