You can run Tailscale on almost anything. Windows, Mac, Linux of numerous flavours, BSD, Android, perhaps iOS (can't remember). It's just install, run, sign in to account and approve the end point to join your virtual LAN, similar to ZeroTier, but slightly better. Very easy and free for non-commercial up to a certain size network.
I think the CloudFlare tunnels server can run on a bunch of stuff too and is quite easy.
I would say if you've installed ZeroTier on Raspberry Pi you could easily learn to do any of the stuff I've mentioned and you should be OK also with installing OpenWRT on your router because it's just another embedded Linux system.
If in doubt about OpenWRT, you could probably install Tailscale natively and run it on a Pi with 3 or 4 commands. Or install Docker then a Tailscale Docker container in not much more.
See here to install Docker on a Pi (very similar to installing on any Linux distro)
Docker is just a host for application containers. The containers are a bit like a VM but they only need the app itself and not a whole O/S image but use the kernel from the host system so are very low resource use.
See here to pull the Tailscale image and run (ignore the bit about building, the pulled image is built already)
Or similar for ZeroTier, see https://hub.docker.com/r/zerotier/zerotier
The main important command is to install and run is (where the string at the end is your network name)
docker run --name myzerotier --rm --cap-add NET_ADMIN --device /dev/net/tun zerotier/zerotier:latest abcdefdeadbeef00
Then to check which networks you're connected to
docker exec myzerotier zerotier-cli listnetworks
If you are still not joined you could run (where abcdefdeadbeef00 is the network ID)
docker exec myzerotier zerotier-cli join abcdefdeadbeef00