Unpopular opinion: IDK why people want perfect interop so much, I have a Mastodon account and a Lemmy account, big deal. We've got bigger fish to fry than this. The formats are different enough that you're better off having separate accounts for microblogging and threadiverse.
Interop for similar platforms is a great feature, but for dissimilar platforms I don't think it's actually necessary just a novelty. Also I think people try to push this on new users as some big, useful, important feature, but I think it only confuses the new users.
Also I noticed most of the time when people complain about ActivityPub interop issues, it almost always ends up being Mastodon's fault lol. Probably because they were early to the party and didn't have to worry about interop and standards much back then. At least I hope it isn't malicious lol.