Hello. Me again!
So I just did the following inside the guest:
# apt update
# apt upgrade
# apt autoremove
The last command cleared off some extra backup kernels. Doing this update/upgrade appears to have fixed the problem. I should probably have done that first. However, the system was not horribly out of date, so I'm not sure if there might be (or was) some other issue.
I spoke too soon...
After rebooting the guest again (I did a reboot as part of the upgrade procedure I mentioned before). I'm back to where I was: The "Fit Guest Now" option is grayed out, and I'm stuck with a low resolution window for the guest.
So it seems that I still need some help/direction here.