I know this is not the best answer since you would probably like me to talk about Nebula, but I have to say that the best solution I have found for setting up a mesh VPN is Zerotier.
It is a very complete solution. Multisystem, very simple but very configurable, fast, etc.
You simply start by creating a network on the public controller (which will generate an ID for that network) and then join the rest to that network and everyone can communicate with everyone (by default, then you can create subnets if you want).
Using the public controller is completely optional (I personally use it because it is convenient for me and because I have few hosts) but if you want you can set up your own controller, I have an article (the bad thing is that it is in Spanish, but if you run a translator you can understand it perfectly) where I explain how to do it without any requirement. If not, you can use ztncui for it.
Take a look at it, you might find it more attractive than Nebula.
By the way, for me one of the great advantages of ZeroTier is that I don’t have to worry about certificates and keys, the controller takes care of everything for you and security is guaranteed from the point of view that each node has a unique identifier.
restic without any doubt. I use it with S3 backend and SSH copy and it has an excellent performance (with copies of years).
Borg I was using it for a while (to compare) and I do not recommend it, it is not a bad product, but it has a lousy performance compared to restic.
Kopia I didn’t know it, but from what I have read about it it seems to be very similar to restic but with some additions to make it pretty (like having ui).
Some people say that Kopia is faster in sending data to the repository (and other people say it’s restic), I think that, unless you need ui, I would use restic.