Android 11 update disabled the ability to install sideloaded APK's

Android 11 update disabled the ability to install sideloaded APK's

This might not be the right place to ask this question but I have been lurking here for a bit and am hoping someone can help me out.

A quick bit of background. I just recently started using Android Tablets. So far all I do with them is to read ebooks, browse the web in an emergency, and, wirelessly access my Xfinity live TV and Peacock streaming service. I have 3 Onn tablets (7", 8" and 10") one of which is Android 10 and the other two are Android 11.

I have absolutely no interest in anything Google and have disabled pretty much all the pre-installed apps based on guides I found here. The only pre-installed apps I use are Files-by-google and Gboard. I use the Nova7 launcher. I do have a google account and access to the google play store but only use it in the extremely rare case I want a purchased app, otherwise I download APK's from sites like APKMirror and APKPure to my PC, validate them using MetaDefend and VirusTotal and then load to the device via usb.

I also have no interest in any cloud based storage.

In general I have all goggle apps disabled. In the rare case I do use the play store I have to first enable Google Play Services and Google Play Store and then once the specific app is installed I re-disable Google Play Services and Store. Also I only go online to use the Xfinity and Peacock apps, everything else I do offline.

So everything worked as I wanted it until at some point I allowed an Android 11 "security patch" to install on one of my Android 11 devices. Immediately I could no longer install any APK file on that device (see attached). Note that previously I could install apps with the Nova7 launcher, the Files app, the Files-by-google app and Firefox, all of which are enabled to install unknown apps. Now all of these methods result in the same error message (see attached). Still no problem installing on the Android 10 device or the other Android 11 device that I've kept from updating.

So does anyone know how I can get back to being able to install APK's? I've followed the standard recommendations of restarting the device, making sure install unknown apps is enabled and reset app preferences and the only thing that I see to do at this point is a factory reset. A factory reset would cause me a whole lot of pain to set things back up to how they were before and I'm not even sure that a factory reset is guaranteed to fix this.

Can anyone help me?