Hey again, good question.
Well I’ve seen the System apps/Apple apps be duplicated in the Macintosh HD/Apps folder as well as being in the System/Apps folder, for that reason I could delete the apps in the /Macintosh/Apps folder to just resolve the issue.
I think in your case, you shouldn’t have an issue at all just erasing your drive, (If you keep a Time Machine back up of course), making sure there is only one volume, then reinstalling the OS and re-migrating your data back onto your Mac:
First sign out of your Apple ID in System Preferences > Apple ID > Overview.
Boot to recovery mode with Command R.
Open Disk Utility.
Click View > Show All Devices.
Erase the top level HDD/SSD/Fusion Drive just above Container Disk under Internal.
The format should still be APFS and scheme GUID, but you can rename the drive, (Usually its Macintosh HD), then go ahead and erase.
Next hit Disk Utility > Quit Disk Utility.
Make sure you’re connected to your network, then click Reinstall Mac OS, when choosing a drive to install it on, choose Macintosh HD Or whatever you named it when erasing).
Once finished, during the setup, you can connect your external HD and choose the option to move data from Time Machine to your Mac, when done, that’s it.
If you would only want to move the necessities of your apps and user data, to avoid storage and performance issues, you can do that too by skipping the migration, then picking and choosing the data from your backup.
However in your case, I know the third party apps and app data are important, so the easiest thing to do would be to migrate from your Time Machine backup. Hope that helps! It’s a process, I know, but most of the time is just waiting for it to complete. Hope that helps!
These are some related articles that may also help with the process:
What to do before you sell, give away, or trade in your Mac - Apple Support
Erase and reinstall macOS - Apple Support
Move your content to a new Mac - Apple Support