IMO a factory reset and final software version install and backup may be a wise thing you would want to do at least 30 days before your update period expires to reduce tablet clutter , speed up tablet and insure longer software stability. You will need to set aside a day to do this and have access to stable High speed internet the entire time .I believe you shouldn't wait till the last week of your update period in case tablet or server develops an issue that might not get resolved in time under warranty. Especially if you don't plan to update and prefer to upgrade to a new scanner in a few years with larger memory and newer Android OS.
I'm not sure if you can even create a working Scan folder backup to a USB drive if the software is split up with BMW installed onto an EXT SD card.
After a Factory Reset,(during update period) the scanner will have to be set up all over again (now would be the time to sell it months beforehand so the factory reset and call in to support for registration change over to the new owner for the information can be done , rather than waiting and selling tool as is after the update period with all your primary email and information still bound to it, where a update would have to be purchased to download all the software over again should he do a Factory Reset out of update period and the backup software for it won't work afterwards.
For a 32gb memory units that you no longer plan to update for the last 30 days of your subscription, you can get everything to fit on the internal drive if you remove the ext SD card and you Install the car Makes and their affiliates with the largest files sizes first starting with BMW.