Is it possible to connect two iMacs via Thunderbolt 3 cable?
I just found information about how to network via Thunderbolt for two Macbooks.  I've yet to verify that it works for iMacs, but it seems reasonable to assume it will, and will perform significantly faster than ethernet networking.
See: https://9to5mac.com/2016/11/22/macbook-pro-thunderbolt-3-bridge-network-video/