Does you Lemmy server use its own SSL? Then shouldn’t you disable SSL on Cloudflare?

That is also a very good idea!

A Lemmy instance for selfhosted software, but each community shouldn’t strictly be about selfhosting said software though.

/c/plex should be just like /r/plex.

I think it would make most sense to collaborate with similar subreddits, which overlap with /r/selfhosted.

/r/selfhosted /r/homeserver /r/homelab /r/datahoarder

These subreddits and more overlap, so to me, it would make sense if these subreddits would come together on the same Lemmy instance, jointly controlled and managed, with each of their own community.

AFAIK Synapse, which you use to run Matrix, is a huge resource hog. It would probably need +4GB RAM.