If I put together a multicommunity and each of them has pinned posts there's a lot to scroll through until I get to the sorted feed. Would it be possible to make it so the pins are ignored in multicommunities and just follow the regular sorting?

Yeah, the pinned posts in multi communities are a scourge lol. Would be nice to be able to turn them off in this case.

Or as an alternative maybe if there is more than x (3?) pinned posts they could be grouped and when tapping on it it would open a pinned posts screen.

Not aware if others know so just as a manual workaround you can hide these individually. Doesn't help if the communities make new ones though, you'd need to keep hiding these as they pop up.

I think this is technically possible but it is annoying to implement. Added to the roadmap. As a temporary workaround I suggest hiding pinned posts manually.