I'd love to find a solution to this! Unfortunately the initial echo was caused by using a lav mic for people in the room. Zoom does a "mix-minus" removing the speakers voice to prevent feedback , and using the lav for the in-room speakers gave Zoom two slightly out of sync audio signals to try to figure out. Fortunately Cramer 53 is a pretty good room for speakers that can project, but it's still hard for some people to hear.