Probably homelab/self-hosted communities, but you'll have to preface that you're focussing on the security aspect than the usability of the application/device. With that said, I'd like for the kind of community you mention to be big and have a lot of engagement but I think it's too niche even amongst niche communities like self-hosting.

With that said, the principles are largely the same. Netsec applies to everything that touches the network, FOSS or otherwise. If you're using Zigbee, you're going to have to read about RF and how to secure yourself (Zigbee uses symmetric encryption from what I've heard and I really don't like the idea). Funnily enough, when I had posed a question on RF privacy here I was ridiculed and downvoted, seemingly by a community that "cares" about privacy.

Yes, there's a lot of hypocrites here. Which is also another reason why you probably won't find much traction for the community you're thinking of. But I'll stop there.

Edit: I completely missed the HA, Node-red and OpenHAB communities, but you'll probably find them in other forums and not particularly active here other than Home Assistant