I do this exact same setup but one thing to add to your answer and be aware of is that syncthing is not a backup solution. If you delete the files on one computer, those files will be deleted on the other synced devices. And accidents can happen.
So, as always, take backups.
Proxmox host. Fedora server vm.
The hardware is a 10 year old Thinkpad. I think it’s pretty clear by my software list that I don’t ask it to do much, but it does so much for me. Like, I wouldn’t run Jellyfin off of this thing. In fact my NAS is 4x8TB drives but I keep it mostly shut off. It’s powered on maybe about once or twice a week for a few hours at a time. I try to batch my activity with it. Like “oh, yeah, I want file X but it’s on my NAS. Maybe later, when I have a need for file Y I will turn it on and retrieve both.”
I can achieve everything I want with even lower spec hardware, but this Thinkpad has a faulty trackpad anyway, which is also how I got it for cheap. I have never measured it, but supposedly it consumes around 6W at idle which is low enough for me.