

ActivityPub is supposed to be a solution to this problem.
As far as I know, Mastodon and Pixelfed are already interoperable, and shouldn’t need a cross poster. Bluesky users can be reached through BridgyFed. Lemmy is only partially interoperable with Mastodon, but this is a result of developers’ choices and not a limitation of the protocol. I can post to all four services, for example.
Mastodon enforces character limit for its own posts, but not for posts coming through federation. Most Fediverse platforms work in the same way, so differing character limits is not a problem.
I don’t know about Bluesky, it may truncate long posts because it uses a different protocol.