The difference between a feature update and a repair install all lies in what version of Windows 10 is already running on the system. If, say, you're running Version 20H2, and follow the directions using the ISO for 20H2, that's a repair install. You have the same version already running as the ISO file would put on.
If, for example, you were running Version 2009, and used the ISO for 20H2, that's a Feature Update.
Since there's absolutely nothing to lose with a Repair Install, if what you did previously was an actual Feature Update, probably via Windows Update (though you could have used another method), it's certainly not going to hurt to do a Repair Install.
For myself, if doing an actual repair install does not fix the issue, what you've described so far indicates a Windows 10 instance that has significant corruptions. I would not trust it going forward even if you do get the issues at hand fixed. My typical statement is, "You can't build a solid foundation on quicksand." There are times when it really makes more sense to go the route that's more painful in the short term to prevent ongoing and recurring pain that could be avoided entirely. That choice, however, lies with you. You are receiving solid assistance from
@xilolee as far as trying to do deep under the hood diagnostics and very targeted repairs.