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.