I very much appreciate all of the effort that folks in this thread have put into trying to get FDE to work. It still seems as if the process is a bit buggy and complex, and I definitely don't want to run the risk of an autoremove completely locking me out. I have been using LUKS encryption on my previous XPS (which was originally a Windows machine) for years, and this is one feature that I don't really consider optional.
Given that I really need my XPS to be stable and I also want FDE, I'm considering:
I have enjoyed vanilla Gnome Ubuntu for the last few weeks on my new XPS, but I've been a happy and productive Mint MATE user on my old XPS for years and wouldn't mind switching back to that (or perhaps a different DE) as long as I won't lose Dell driver support.