tavu

Usually they're normal x86 PCs with nothing unusual about them so just your Linux/BSD distro of choice. You can look up the processor model to see what crypto acceleration it can do, or see if there's any wireguard benchmarks available.

Some have interesting processors like PowerPC, or other strange hardware, but avoid them unless interesting is what you're after.

You have the pi, give it a go.

If it's inadequate then i'd recommend a used fanless thin-client type PC, such as a Wyse 5070, just make sure it comes with PSU and a few GB of RAM and SSD. And check reports of how much power it uses at idle.