Remove unwanted startup checks and lag

Is your feature request related to a problem? Please describe.
"Your system is up to date" ... No it is not!
In order to have control over when the net or other resources will lag, I have preferences set with intent to manually update, and mintupdate not set to launch at login... But when I run mintupdate, there is a spinning beachball delay, presumably while the stale apt lists are parsed. It also seems that Flatpak updates are checked for during this delay without my hitting the "Refresh" button.

Describe the solution you'd like
At launch, if all preferences are set off, except the one to include Flatpak updates, the option to refresh should be immediately available. Further, if a preset period has elapsed since last refresh, the main body frame should not misleadingly say "Your system is up to date", but rather have a message that the package lists are stale (maybe even an indicator of how long ago the last refresh was) and point the user toward hitting "Refresh". That period ought to be a preference, and could also cause a pop-up reminder if mintupdate is running at the time of the threshold.

Describe alternatives you've considered
Sitting waiting for it to start, then having to remember that I've not hit refresh, as mintupdate reports "Your system is up to date" (when it is not) is suboptimal at best. I now tend to use apt update && apt upgrade in an xterm, then use mintupdate solely for Flatpak and kernel.

Across my whole mess of VM's, remote supported, and physical installs, I'd really like to see this fixed.

Start by tracing mintupdate's startup path, preferences, package-list refresh, and Flatpak update checks, especially the Refresh action named in the issue. Done means startup no longer performs unwanted checks, stale lists are clearly reported instead of showing "Your system is up to date," and the requested refresh-period preference and reminder behavior are defined and tested.