It turns out possibly to have been related to BitDefender. Not sure, but after installing Big Sur I noticed my MBP was heating up. I use ‘Macs Fan Control’ thermal monitoring app. So I checked Activity Monitor to see what caused the processing overhead, and it was BDUpdDaemon from BitDefender. BitDefender is a good app, but a few others have had this problem with overheating or slowing.
They have a nice Uninstaller (whew! It’s in the application folder; there are tedious ways to do this manually, but the uninstaller works like magic). I rebooted, and SketchUp worked very well, almost restored.
I still have some trouble with Push-Pull actions collapsing into 2D objects after I type in dimension. A brief report came up on the screen at one point but was ephemeral – it said something like “VCB error caused operation to fail”. something like that.