OK, nobody answered, but by combining advice from several different posts in different places I managed to brute force it down to 800 KB. Here's how I did it, in case someone else has the same problem.
First, I ran the following in a CMD window brought up with admin priviledge:
takeown /F C:\$Windows.~BT\* /R /A
icacls C:\$Windows.~BT\*.* /T /grant administrators:F
rmdir /S /Q C:\$Windows.~BT\
There were a lot of failures, but it got the folder size down from 25 GB to 12 GB. Encouraged, I restarted in safe mode (PC Settings, Recovery, Advanced, Restart Now, ... various options ... safe mode), and then did the above again. Again there were failures, but now it went down to 800 KB, which is entirely acceptable.
So go figure. I have no idea what the problem was or how the "solution" worked, but it got me pretty much where I want to be.