Syncthing supports introducer devices. If you mark a device as introducer, you'll automatically add all devices the introducer knows to your list. So basically adding a new device to an existing group requires everyone to agree on one introducer, and everyone will add the new device automatically.
NAT traversal and relay servers exist too. So seems it's pretty much on par with Resilio, plus open source.
For my specific use case, there's probably no device that could be an introducer.
It's all PCs, none of them online 24/7.
Besides, setting all that up requires way more technical knowledge than most people have. Sure, I could do it I guess, but explaining Resilio configuration to complete non-techies is hard enough.
NAT traversal and relay servers exist too. So seems it's pretty much on par with Resilio, plus open source.