Just a note to let the gang know that a iMac 17,1 late 2015 iMac is running Diablo 3 just fine without really any issues using the first Big Sur public beta (11.0 Beta (20A5343j). This is essentially the Dev beta 4 release of Big Sur.
10.15.6 was getting a bit buggy with a kernel memory leak effecting some computers, it was discussed on the VMware forums.
This needs to be installed as update to a 10.15.6 APFS volume preferably. Its a big installer (12 GB) and needs adequate free space to allow the install to compete.