We are (like everyone) on matrix.org now but realize we need to move eventually.
Consider moving to another open registration server too. Find one that supports Element Call
do I need to pay for a domain still?
If you're gonna selfhost, you should purchase a domain for proper federation with the wider network. IP-only servers are possible, but they are generally banned in most rooms due to antispam. Same with dynamic DNS domains
Unless it really is easy enough to do it on a synology nas for text/voice/screen share...
You'll need to integrate a Matrix homeserver (I recommend Continuwuity.org, much lighter than Synapse) and Livekit (the software that handle Element Calls). It's not particularly easy so maybe consider managed hosting beforehand, too