So far this is the best one I've found. A few discord communities were good but like you said seem to have quieted down (though the one for Fully Automated is getting active again, and we have some general solarpunk discussions, but it is based around a solarpunk TTRPG rather than solarpunk in general).

I've heard there are some solarpunk matrix channels but I haven't sought them out yet. So far this Lemmy instance, Mastodon, and the subreddit are my go-tos. There's also some kind of federated discord alternative out there but I haven't tried it out yet.

For what it's worth, discussion posts seem to do just fine here - the place doesn't have to just be a link aggregator. I've posted a bunch of questions over the last year while looking for input and suggestions for my solarpunk art and fiction and people have provided great advice and I've had some really nice conversations. It's a good community.

Maybe try to be a bit more specific? Solarpunk is an umbrella term for a worldview that puts together many things widely discussed elsewhere: sustainability, low tech/open source, anticapitalism, cooperative economy, rewildering... All these more specific terms have deeper discussions going on. Solarpunk is basically a modern synonym for "utopia"

The #solarpunk tag on Mastodon (and compatible Fediverse microblogging software) has some interesting stuff sometimes, but it's just not a very big scene so to say.

There are a few other language Solarpunk communities on other Lemmy instances, but they are not very active.

FWIW: I’ve tried bluesky and not found more than a handful of genuine solarpunk accounts, so that’s off the list of possibilities.