So, now the unexpectedly arduous task of getting to a point where I can even install Bazzite on the ROG Ally...
Despite turning off Bitlocker, disabling the virtual page file, disabling system restore, cleaning all temp files, and defragging the drive, Windows will only let me shrink 70GB of the 350GB that are free.
(╯°□°)╯︵ ┻━┻