Ive always wondered. - Should I be adding relevant hashtags to Lemmy posts related to the subject of the post to make the content more findable on other Activitypub services?
You can if you want, but Lemmy doesn't translate them, like Mastodon does, and you might find them conflicting with the header Markup.
That, and Lemmy users used to a more Reddit-like structure of posting might get rather annoyed by it, since it would clog up the feed with unhelpful information.
I can only speak for how it shows up on Mastodon, but over there any hashtags we try and add here just show up as plain text and don't show in the actual tag feed, so it does nothing for discoverability.
I'm not sure but I believe from Mastodon's point of view, you are already replying to a group that is the community you're posting to. Or tagging that group in a way. So in that way, it is discoverable by looking at the group, at least in theory.