What about censorship circumvention? Is there specialized DERP to DERP communication, that bridge over internet edge nodes doing DPI on QUIC?

We do not use DERP. But yes, relay to relay communication is something we want to look into in the future for some use cases.

As of now relays are completely self contained and pretty dumb. The protocol does not require relay to relay communication, which means that the relay code can be relatively simple.