After a clean update to Catalina (iMac 2018) and installing Steam and Eso anew (so these and Launcher are up to date for sure), I had the spinning problem.
I went through the usual procedure (Security Privacy etc) I'm already familiar with, plus rebooting the Mac. To no avail. The only thing that worked was to start the game directly from Launcher, bypassing Steam.
I think that the Steam client is where the problem reside, anyway, anyone else has had this same problem (that is, Steam related to spinning), and if so, has solved it somehow?