Hi, I let the others reply to your question. But I suggest you to do at least the following:
- Boot without SIM card and without internet conection
- Unlock the bootloader
- Boot into fastboot
- Boot into TWRP without installing it
- Open a shell on your smartphone
- Use
ddin order to backup the following partitions:
- aboot_a
- boot_a
- cmnlib64_a
- cmnlib_a
- devcfg_a
- dsp_a
- dtbo_a
- fsc
- fsg
- keymaster_a
- lksecapp_a
- mdtp_a
- modem_a
- modemst1
- modemst2
- product_a
- rpm_a
- sbl1_a
- system_a
- tz_a
- vbmeta_a
- vendor_a
- DDR
- apdp
- config
- devinfo
- dip
- dpo
- frp
- keystore
- limits
- logdump
- mcfg
- misc
- mota
- msadp
- oem
- persist
- prodinfo
- sec
- splash
- ssd
- syscfg
Doing this should guarantee that as long as you are careful to keep your system untouched in at least one of the two slots (you can experiment on the other), you would have a very hard time bricking your phone irreversibly. And if you are unhappy with some update, you should also be able to actually make a real “factory reset” of the phone.
Probably you don’t actually need to backup as much as that, but I preferred to keep on the safe side.