correct me if I'm wrong ... but I guess the software update saves some files for Flash Restore on your drive, so if you format it u won't b able to restore to your old firmware..
so my suggestion is:
restore to your old firmware through SE update website
restore your files from the backup file
check if you still have your phone crashing
if it's not ... then stay on this firmware as I didn't notice any big diferences after update, otherise ...
update again
format
restore your important files (contacts, mails ... etc)
reinstall your programs
(it's take a long time if you go with this option)
good luck😊