Pamac-checkupdates seems to be hyperactive

If pamac-checkupdates is running then yes you are using pamac’s checkupdates functionality.

Open the GUI preferences and disable the check

this means another process has locked using /var/lib/pacman/db.lck - if you are sure no other process is running - you can remove the lock manually.

See → Pacman troubleshooting - Manjaro
See → [root tip] [How To] Mitigate and prevent GPGME error when syncing your system

You can disable by editing /etc/pamac.conf if that is easier.