I haven’t seen prituni before so, thank you, I will have a play.
Zerotier is actually free. It is only the online controller interface that is commercial at this point (for more than 50 devices). Setting up your own controller is simple: https://key-networks.com/ztncui/ Or even here on NS (in case you missed it): Zerotier (controller?) support
I have it implemented at a few locations and love it. Even just for my own support services being able to connect to other machines/servers in my zerotier network without the need to open ports on routers etc. I also use OpnSense as a simple router for that network - it has a built in process for the zerotier interface making routing it quite easy.