Sitrep:
Disaster.
The goal was to see if I could install Microsoft Office 2021 on a no-name brand 8" Windows tablet. The tablet's connection through WiFi was too slow and the installation kept timing out. I eventually copied the image file onto a USB thumb drive that has both a standard USB-A connector and a microUSB connector, and installed from that. It took a VERY long time, and I had to re-start the installation once, but Office Pro 2021 did finally install. Alongside Office 2010.
BUT ...
When I entered my product key, it spit out an error message asking for the product key associated with the Microsoft account through which I made the purchase. But I didn't buy it through a Microsoft account, I bought it from a third-party vendor.
The other issue is that the installation didn't ask me where I wanted to install it, so it went onto the C:\ drive -- leaving me almost no space on that. I want it on the D:\ drive, which is a 64 GB microSD card that has plenty of headroom left.
So I decided to just undo the whole mess. I called up my restore utility, only to find that the restore point I had just created ... does not exist. And, of course, I hadn't been able to clone the drives, so that left me with the only recourse being to use the Windows Programs > Uninstall utility to get rid of the newly-installed Office 2021. I ran that, and it left me with a huge mess of stuff in the Windows registry, which I used CCleaner to remove.
Office 2010 was (and is) still on the computer but, when I went to start Word, it had to reconfigure itself, and then I received a half dozen alerts about some font (identified only by a number, not a name) not being "registered." I skipped those, and Word finally opened and seems to be running. The same thing happened with Excel and Powerpoint.
I am now (I think ... and hope) back to where I was before embarking on this journey of discovery. I think this is my clue that I should be content just to leave Office 2010 on the tablet. If I need something newer, I'll install SoftMaker Office. But I will have to contact the vendor and ask why Microsoft wouldn't accept my product key.