Depends on your comfort with CLI tools. Here's the process (assumes Windows):

1. Download and extract platform tools
2. Add that location to your PATH

• Win + R, type "cmd", enter
• set PATH=%PATH%;C:\your\path\here\ <- Temporary, just for the current session
• setx /M path "%path%;C:\your\path\here\" <- Permanent

3. On your device, go to Settings -> About and look for Build Number it can sometimes be buried in Software Information

4. Tap Build Number repeatedly until a message appears You are now a developer

5. You should now have a new Developer options menu item somewhere in your settings. Sometimes it's top level, sometimes it's buried under Additional Settings or Advanced Settings or the like

6. Make sure USB Debugging is turned on

7. Connect the device over USB

8. Back on WIndows type:

• adb devices
• You might get a popup on the device asking if you want to allow USB debugging. Select Yes, and run adb devices again. You should see your device listed

8. Download the APK of the app you want to install (AAB files are a PITA, but can be installed too. Try to get APK files though)
9. Install with:

• adb install C:/path/to/app.apk <- if adb devices only returns one device
• adb -s <device_id> install C:/path/to/app.apk <- specific device

You can install updates the same way, just download the updated APK and add the -r flag adb install -r C:/path/to/app.apk

More useful adb commands

No it's not. Not because of that it's too difficult but it's too much work on a weekly basis just to update your stuff.

I have no rooted phone because I have grapheneos but it should be possible to do it directly on your rooted phone with shizuku et al which wouldn't be that bad.

There is also Shizuku that might be utilized in der Future to install apps (Im just speculating here)

Yes it is.

It will probably be something similar to ios altstore. You would have an app which has a list of your non-playstore apps and repo for their releases and would notify you for upgrades that you would manually have to install (for example I suppose obtainium will implement adb), the setup will be annoying but far far less cumbersome than apple where you have to reboot your device multiple times self sign ipas and refresh once a week otherwise the apps don't load at all. This iOS experience is awful but still doable and non-dev. Android will have a better experience for sure.

Is this manageable for the non-dev by chance?

Not really.

I've not been following things super closely, but the idea would be that each user would get their own developer key and then locally compile and deploy whatever apps they want as though it were a project they themselves were working on. The first bit is not too dissimilar from how a lot of people with XBOXes made dev accounts to install emulators. But the latter is going to get real messy and REAL compromised REAL fast as people just use third party tools and binaries that will inevitably be compromised.

I’m feeling a dumbphone alt may be the only viable path

It really depends on what your use case is. If you actually just talk to people on phones? Uhm... I am not even sure where you would find a dumb phone at this point, but that will probably work for voice calls and SMS using just your carrier and MAYBE wifi. But anything that involves apps, which is a shockingly large part of the world, will be a mess. Some you can (and should) do workarounds (banking apps, for example) but others you are kind of up a creek since your options are to use a modern phone or not be able to (for example) see your kid's daycare schedule.