The problem is
likely to do with a combination of the OP
- choosing to encrypt the drive at install time and
- choosing inappropriate Settings for Timeshift
If LUKS encryption is used for a drive that has Timeshift on it, /boot must remain unencrypted in order for it to reboot successfully.
From the images the OP supplied, I am guessing that he has his Timeshift settings under User as either including his Home folder or partition, or have the options checked for Include Hidden Files.
The encryption perhaps has not worked as desired, and all of its content is being stored under
/home/.encryptfs/matthew/.Private
The outcome is likely that the two snapshots have taken up a larger than intended amount of space.
The first snapshot, with a flag of O (On Demand) was taken by the OP on 1 April.
The second, scheduled to Monthly, occurred at 3PM 1 May. This one likely "broke the bank".
If the OP follows his plan
... and modifies his Timeshift settings, he should be OK in future.
He also said
... which is likely a good idea.
@CataclysmicGentleman if you let us know here when that is accomplished, I can give you better instructions on Timeshift at my thread
https://www.linux.org/threads/timeshift-similar-solutions-safeguard-recover-your-linux.15241/page-32
Cheers
Wizard