Ooh, just this week I started toying with a fork of takahe to see if it could be extended beyond microblogging. Some questions:
Where have you found a proper documentation of Lemmy's API? All I found on their website was the documentation of the Javascript SDK. If you have something like a Swagger/OpenAPI description of the API, it would help immensely.
Why the mix of Java and Go?
You mention a new API. Is there any chance that Sublinks could be developed as a more "strict" ActivityPub-compliance system? For example: would it be possible to architect the new features in a way that it only relies on the actor outbox/inbox?
A bit more difficult question: the reason that I was looking at Takahe is because it's the only AP server (that I know of) which supports multiple domains being served from the same instance. For someone providing "managed hosting" like me, it would save me a lot on resources to have one single server for multiple customers instead of having to spawn a new Mastodon server for every one that wants to have their own domain. Is there any "killer feature" on Sublinks planned that you'd say could warrant yet-another tool? Why not contribute to Lemmy instead? Or, if the devs are more experienced with Go, why extend/contribute to GoToSocial?