What is the recommend wireguard mesh solution where peers can easily join and leave and shortest connection paths are automatically determined?

I want to setup a way to access my mobile home office subnet (behind cellular which doesn't allow incoming) and two other subnets sites and some mobile devices which come and go.

Have you taken a look at Nebula [0]? Might fit your needs. It also uses the Noise Protocol Framework but adds the bells and whistles on top needed to synthesize an overlay network like you want. MIT licensed too fwiw, full self hosting. Worth a look at any rate, WireGuard is much lower level though I'm sure it could be built upon for the same purpose.

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0: https://github.com/slackhq/nebula