Probably not a RasPi if you're going to open it up to many people, but I'll admit I'm not a hardware guy. My guess is that it'll be a little heavier and slower than a standard relay, so it would be for somewhat specific communities and not tens of thousands of users.
Discussion
Used Intel NUCs have a lot more power for the price, I'd check out eBay. They can be a bit sketch though, the one I have I need to keep switching display ports for some reason I can't quite figure out yet.
If it's one with two DisplayPorts or HDMI inputs, I noticed the same thing, loses signal until you unplug the replug into the other input port.
Personally I rate Asus Mini PC's they have AMD CPUs.
I'll try that for my next one, I can hardly get into the bios on this POS.
Well worth it. Barebones kit (bring your own SSD + RAM) is around $250 brand new. Sure you could grab cheaper deals off eBay too. You get much better build quality than the NUC and a Ryzen which kicks the shit out of Intel these days.
Got my node running on one plus a Tor Nostr relay, Nextcloud, Robosats and a few other little things. Doesn't break a sweat.