Lemmy has cool features like inserting pics in post body and being very actively developed. We will get federation, an app and lemmy/hexbear's development would be accelerated because of the combined resources...
Lemmy supports display names which can be used for pronouns until hexbear devs upstream that and it also supports avatars and banners.
I'm aware that hexbear devs wanted to leave Rust behind and this way they can just upstream the features that lemmy doesn't have and leave development to lemmy's devs if they want to.
Getting back to lemmy is a lot of work but will probably be very rewarding in the long run.
I also think it'd also be great for hexbear to have posts from other comms outside of hexbear... we don't want to turn into a small closed community.
Nothing about the way the site works should be incompatible with ActivityPub, although the hexbear API is no longer compatible with lemmy. Someone just needs to code the federation, since I guess it's not portable from lemmy to hexbear anymore.