The popup picker and the inline picker are both external js libraries lemmy-ui uses, with varying degrees of customizability. The inline picker uses "tribute", a JS library meant mainly for things like @mentions, and the popup is a component called "Emoji Mart". I have looked at the search code for the inline picker and found it is pretty simplistic so I'm not shocked that you'd prefer the emoji mart (I haven't reviewed that code).
Totally agreed about the emoji mart popup inserting at the end. I don't know how easy or hard that change will be to make but it would be a welcome one for sure.
I also think we could still improve loading performance on emojis, though mako's lazy loading change helped a lot. They don't generally change, so they can be cached pretty aggressively, it's just been challenging to get that working right.
Lemmygrad is a good bet, though official announcements of this sort should also be posted to https://toots.matapacos.dog/@hexbear
Edit: I see FloridaBoi beat me to it.
Announcement post here with a brief explanation of what happened