@Fiat Lux - ASIA 

I've never tried installing and running multiple versions of QB at the same time. Didn't know you could do that. Doesn't that mess with the company files? I know sometimes the company file gets converted to work with a different version. Surely, you're not duplicating the entire company file to use a "second copy" with the other version are you? That would not be prudent.

You suggested I install the trial version for QBD 2021 Enterprise. Where is the best place to download that?

And FYI, for anyone else following this thread, I just now opened QB and it took 5 full minutes to just load the app itself, no company file at all at that point.