On Discord, you cannot host your own server, and you cannot use any third party clients (without the threat of being banned).

You can host your own Matrix server, either on physical hardware, or a generic virtual machine you can rent from any number of ISPs. There are over a dozen compatible third-party clients (though many lack full feature coverage).

In summary, Discord is strictly a service. Matrix is a tool you can apply however you see fit.

Support for groups (i.e. communities on Lemmy) is coming to Mastodon sometime soon.

I feel like PeerTube hasn’t broken through yet in the way Mastodon has, and Lemmy is kind-of broaching on. Mastodon itself is heavy for what it does. I need 8GB of RAM, >600GB of storage, and 2 CPU cores to run a 100 person instance. Lemmy is leaner (as well as some microblog style alternatives to Mastodon like Misskey / Pleroma). Peertube, on the other hand, can only get so lean. Hosting video content is orders of magnitude more intensive than hosting a text-based message board. It is much more costly to do this, and to compete with platforms like YouTube, it is not sufficient for just spin up a single instance. You also need to work out CDNs, caching, load balancing, etc.

Like Jack said, I’d just find an instance you vibe with and post stuff there, but it will take a lot of resources to grow the network as a whole.

AFAIK the problem with resin printing is vapors, not particles. A respirator may help, but it is no substitute for proper ventilation.