Sounds like you’re talking about a Mesh VPN.
Syncthing doesn’t need a VPN to function - in fact you’re better off not using a VPN as it’s own rules will see the VPN as a LAN connection and sync data across it when your Syncthing rules exclude using your data connection. Maybe that’s what you saw with Netbird’s data usage. I’d be really surprised if Netbird itself used any significant data.
I have about 20 sync jobs per phone - some are allowed to use cell data (photos), others aren’t. When I enable Tailscale on my phone, Syncthing will try to sync all the jobs because it sees the VPN as a LAN.
I run rooted and use a firewall and block VPN there for Syncthing to prevent this.





As others have said, unless you’re going to be using those CPUs a lot, you probably don’t need the capability.
I run an old (2019) Dell OptiPlex SFF desktop as my server (I also have an ancient NAS). It runs ESXi just fine, with 2 Linux VM’s and 4 Windows VM’s, with 48GB of RAM.
At idle it consumes just under 20 watts. Peak is 80 watts (limited by the power supply).
Those VM’s all run fine - one is for file services and Jellyfin, one is for dedicated DVD ripping and video conversion.
Even when that VM is converting videos, everything else is responsive. Never get a lag on Jellyfin.
Now imagine how much more performance your server is capable of. Many simultaneous VM’s.
Oh, and it’s a really quiet machine.
I’d sooner have this or multiple mini pc’s than any kind of commercial server hardware. Completely different design approaches - servers are designed for running 24/7 with maximum cooling capability along with max performance. Power and noise aren’t really a consideration.