I had to do MTU tuning on macos on the ZeroTier interface (find your feth name via ifconfig)

# Replace feth1234/feth2345 with your active interface

sudo ifconfig feth1234 mtu 1400

sudo ifconfig feth2345 mtu 1400

..and for working with Windows peers, manually "Orbit" the Windows Peer as well as adding a direct routing hint for the internal ZeroTier IP. ZT definitely takes some effort for tuning.