I've upgraded a bunch of unsupported Windows 10 PCs to 11 successfully.
Some of them were Legacy boot. But one Legacy boot didn't boot. I had no idea what to do. I fooled around with it until I just gave up and reinstalled to Win11 with bypass tricks. Later I realized I just needed to raise the Windows Boot in BIOS to the #1 position.
I say all this because it changed my approach. I've run into a couple Legacy boot Win10 that should upgrade to 11 with bypass but they are used every day and would involve a lot of setup if reinstall is required. The simple answer for now is to sign into a MS account and subscribe to a free upgrade extension.
I remembered the old WSUS offline update program I used in the past and wondered why it's never mentioned as a solution. It is now a community gitlab project. I've downloaded it to check it out. It may be an answer if and when the MS account subscription is no longer available.
Here is the project website: https://gitlab.com/wsusoffline/wsusoffline/-/releases
I plan to put the suggestion for use from Gemini below. Along with my own experiences, of course.