The thread you linked to is the exact same one I found and last night. Is the broken apply to folder view the reason why grouping kept coming back after I checked that box? An article I found told me to disable grouping and then check the box that says apply this view to all folders and subfolders, but when I did that grouping was turned back on.
Based on what you've said, that article was incorrect.
The purpose of the checkbox labelled "Also apply this template to all subfolders" is to change the current folder, and all of its subfolders, to a different folder type. For example, in theory, you could use that option to change a folder tree from type "General items" to type "Music". That function is broken in Windows 11. Or to be more accurate, that function is broken in the Windows 11 Explorer. It still works in the Windows 10 Explorer which can be used on Windows 11 (with a simple registry entry).
But all of that is moot because the function you want is the button labelled "Apply to Folders". The purpose of that button is to update the template for the current folder type and apply that change to all folders of the same type. So, for example, the Downloads folder is a special folder type named "Downloads". When you set Group by to "(None)" and do nothing else, you're only affecting one view of Downloads (aka one "namespace"). To set all views, for all folders of type Downloads, you would need to also click the "Apply to Folders" button. BUT, as I previously mentioned, the "Apply to Folders" button does not catch everything and there are certain situations where it doesn't work at all.
I didn't write WinSetView for nothing. If there was a built-in function that would let you change the folder view defaults to your preferences thoroughly and reliably, it wouldn't be needed. So, I recommend that you use WinSetView and just take your time to follow the steps in the quick start guide, so you get the defaults that you want.
If you don't want to go that route, then just note that the "Apply to Folders" button is all you've got and the "Also apply this template to all subfolders" checkbox is irrelevant to the problem of turning off grouping for the Downloads folder.