After replacing everything (ONT, Eero 6 Pro unit, cables - everything but the fiber), I was forced to give up and ask Sonic to send me the original Eero setup. Everything is now great. Sure, I miss the speed but I have yet to learn to avoid "bleeding edge" and "beta" technology.
I was getting greater than gigabit speed directly out of the ONT and even out of the Eero 6 Pro but downloads as low as 38 megabits on wifi, if anything at all. When wifi worked it was great but that was infrequent.
Eero 6 Pro - shipped before ready.