Define great. Tailscale doesn't even run Wireguard on the kernel level, but in user space.

Can I run OpenVPN configs on it and use it as a roiter

With hardware acceleration.

Computing power isn't just a general quantity. Networking devices have dedicated chips in them to perform various parts of processes. (Encryption, decryption, encoding, decoding, compression, decompression, etc.)

That's hardware acceleration. There are chips that are super efficient and powerful but they can only do that one thing.

That's fine if you know exactly what the device is going to be for, so you can put in the exact chips it needs to do only what it needs to do.

Sell it and get something more suited to the task instead of trying to shoehorn it onto a pi.

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.

Can you just get your own Router and use that instead of the ISP one? Then you can flash whatever firmware you want on it and you can run the openvpn/wireguard client at the router level. You won't need to combine the Pi with it.