I would advise against updating your phone, especially not through the system updater.
You should read a guide on updating with TWRP for when major OTA updates release and you want them
To remove the annoying notification, and to prevent it install
Disable Application and disable the
Software Update system application.
There are plenty of cool applications, such as
AdAway (Root),
Swift Backup
For XPosed, it still lives, AND bypasses safetynet. As you mentioned, it is called
LSPosed. Try it out, it is incredibly nice. You can find some more modules on there
I would advice you install the
Magisk Manager for Recovery Mode module. In short, if any module causes you to reboot, you can disable the module from within TWRP, which is incredibly useful.
For a Magisk Repository, insall
Fox's MMM, which is a huge repository of Magisk Modules for you to install.