Is there any manga/manhwa/manhua(... or any other form of media really) where the story starts like a couple of years or something after the isekai event? Like the teenaged mc got isekaied, beat the demon lord/did whatever they were supposed to do, found no way home, settles down in the new world and the story starts a couple of years after that.

~4400 manga completed
~1700 ongoing...

I don't know of anything that exactly fits your criteria, but a couple of figures in Saihate no Solte are revealed to have been isekai'd from Earth to the world of the story. The manga's still fairly early on in its run, so it's uncertain as to how much this will ultimately play into the wider story.

Edit: I don't think I initially made it clear, but the isekai'd characters in the story have been living for years in their new world.

Hero has returned

In the Land of Leadale kinda fits this, MC gets isekai'd to her favourite MMO on her deathbed, but 200 years have passed ingame. MC was a high-ranking player before she was isekai'd and retains these abilities.

Does it have to be isekai? Sousou no Frieren is not an isekai but is set many years after defeating the demon lord.

Does Uncle from another world count? It's sort of spans the time in the other world and after.