It's done, took about 4 reboots & 2 hours...
Everything looks good & seems to be working quick & well. Thanks for the encouragement, it might have been weeks before I did it otherwise. I was looking for some sign of progress at that Known Issues site. Also, I was thinking the compatibility checks it did would only happen if it came in through Windows Update. But clearly I was wrong about that. I'm overjoyed I installed it.
The .iso ran well off my external Seagate Slim Red 2TB HDD which plugs into a USB port. I guess that USB problem is just for some flash/thumb drives. Or can it be the .iso would work off them too? At various points in time, I did keep my .iso's on the internal HDD & even on OneDrive. But I didn't want them accidentally ending up in my Windows system image backups.