Thanks. I'd already tried most of what you've suggested. In the spirit of giving as much info as possible, I'd just started to write out a more detailed information list in the format suggested in your link. But.... I've finally got there.
The problem was that I'd installed the Insta360 Pro extension for importing 360 footage. This is what was causing the massive & constant hangs. After searching a bit, it seems this has been an issue for a number of users.
So, my strong advice for anyone who purchases an Insta360 Pro / Pro 2 is DON'T INSTALL the Insta360 Pro extension for Premiere, as it will totally kill performance. Just use the separate stitcher software.
I was able to uninstall the extension by just deleting it from the C:\Program Files\Common Files\Adobe\CEP\extensions folder.
Nevertheless, thanks for your sound advice.
Yep, that is why Step 2j is on the list. Glad you got things sorted.