(quite interesting, even though you @ me, i didn't get notif also)

re: akkoma - ya it's a pleroma fork actually. can't quite remember if there was any technical reason for it, it was mostly because the pleroma dev+main instances was infested by nazis and rightwingers and lolis. iirc when i joined in nov 22, that was the "lore" that i picked up.

of the *keys, misskey is the original flavour but the japanese founder dev is apparently a bit of a lone wolf and so the dev for that became quite slow. And I think there was fallout when he, in running the japanese instance for it, wouldn't commit to moderating against homophobic speech or protecting explicit support of lgbtqia. So there was a big break from then on from there. The rest are forks of misskey, but you've pretty much named the major players:

• foundkey is a fork by this swiss guy. Apparently decent but is by this one guy, so that could be an issue.
• calckey is by this german guy I think. This one is going to go on relaunch on the 19th with new name etc. Supposedly it's because the fork has branched out enough it's its own thing, but idk. Sounds cool though, another german dude to go toe up against Eugen's Mastodon. I've noticed a lot more on the Black Americans side are beginning to rec Calckey because of all the functionalities you've also noticed (I'm on a walled-up instance for BIPOCs for example, that runs on it). Just because of what I perceive to be real good prospects in terms of userbase and with it development and funding support, I recommend Calckey, and--
• hajkey (it's swedish for shark, so "hai-ki") - this one also another difference in ideology. The people behind blahaj.zone have been key contributers to the Calckey code repo but they fell out over the growth mindset with the soon-to-be-renamed Calckey so this is a soft fork. So they'll stay within the calckey development cycle. (but who knows how soft it would be; one issue i'm sort of understanding is how resource intensive it can be as an instance grows, especially on the antenna function - btw, my favourite fedi function when it comes to scouting posts from other people.) My kitajagakita alt is one awkward.company, that one runs on this software.

re: federation - the problem is normative centralization - mastodon is the main dog especially in the western and western-facing userbase, so lemmy and *keys and *oma devs do try to make sure the federation works well there. But Lemmy is already a little janky, and maybe that's why it doesn't really play well with the foundkey yet (esp if the fk dev has enough on his plate). try setting up on an instance on calckey (not the main one, they're having continual performance issues) or hajkey and see if there's any improvement from last month (even kbin wasn't federating well with calckey).

LONG POST IS LONG