If the only thing that is running on my opnsense box is opnsense (and related plugins), is it advisable to periodically run sudo apt update sudo apt upgrade if I am already periodically checking for updates from the dashboard/lobby and applying them?
No, because those commands are for Debian-based Linux systems, not FreeBSD/OPNsense systems. Just use the GUI
thank you
GUI and console menu tasks indeed take care of proper updates and sequencing of internal "pkg" commands (the "apt" of FreeBSD so to speak).
Cheers,
Franco