Hey, I've got similar storage and bootloop problems but somehow It was fixed successfully.
Got iPhone XR, IOS 13.6.1, data was backed up only to several months ago. [br]
Couple of days ago I've got a notification about full storage, tried to delete several files, applications and videos, but it was not seemed to be actually deleted. Thought a restart would help, and got into the boot loop situation. Read online and was very worried about the potential data loss.
First tried using 3Utools "iTunes flash" - "retain user data" option with the same IOS installed (13.6.1) but got error 3002 in about 10% of the process. Tried using 3Utools "iTunes flash" - "retain user data" option with newest IOS 14.2 and got another error with a frightening message ("The drive will be deeply formatted after the reboot to prevent the recovery of drive data"). Tried the same with 3U tools "easy flash" - "retain user data" and got similar message.
So I stopped using 3Utools, and started using ITUNES (latest version of 12.10.10.2 from 22.10.2020). Used Apple's manual for an Update https://support.apple.com/en-us/HT201412 , was patient though nervous and almost the whole process seemed fine, no error 14 was received. Only in the end of the update process got on the iPhones' screen a frozen white background with apple logo, but heard notification sounds from the device. Made a force restart and the device got back to life, with the latest data used before the bootloop, and with the updated IOS of 14.2 instead 13.6.1!
I'm not sure what made it work differently than other reports here. Maybe the latest ITUNES version, or the fact that I actually had about 3-4GBs free in the device.
Not responsible for the stuff I've reported here. Good luck!