You definitely should try recreating the registration account as described in that post I linked (it's option #3).
After I posted I looked back and there were 3 or 4 similar threads not just the one I linked. One was exactly the same as you describe as a switch in Certificate Authorities.
It likely is that the registration connected to Sectigo doesn't get fully cleared out when switching CA's
The rate limit is not related to the number of names. It means the ACME Client (AutoSSL) has issued 301 authorization requests and has not even tried to fulfill them. It's just a wrong API traffic flow and is why there is a limit to protect LE from mis-behaved clients.