From OP:Just to avoid any confusion... you mean install the apk. Then what? Maybe this is where I am making a mistake.
11. Reboot into System, allow OS to settle for a few minutes
12. Install MM 5.8.3 APK (like a normal app, not from within Magisk Manager)
13. Reboot into Recovery
14. Install Magisk 16.7 (1674) zip [must use the 1674 version],
15. Reboot back into Recovery
16. Reboot into System
After installing the apk (step 12), reboot into recovery (step 13), then flash Magisk 1674 (step 14)
The process is pretty cut and dry. If you're following each step as listed, with the exact same files as listed, and it's still not working: I'm not sure where to go from here.