What you are wishing to do seems to be what TWRP does: complete system backup that you can restore nearly identically after a full wipe. Unfortunately this requires:
- to unlock the bootloader of the phone, with all consequences it implies. This is anyway necessary for any backup solution for apps and the system that isn’t directly baked in the system, such as Google backup or Seedvault.
- TWRP to be able to decrypt data, which is quite difficult for devs to achieve and isn’t the case on most devices now (but it might be achieved for FP5, I have hope). Nevertheless, I wouldn’t advise you to use TWRP for system backups except if you know well how it works and which partitions it backs up. With my FP2 experience, I had quite a few cases where it was very useful, but also others where it failed.
- to still have access to recovery, which isn’t always the case when tinkering fails. And then you’d just be able to flash the stock OS from fastboot and that would be the way to go.
These apps require root (best is from Magisk), but this again has security implications and may brick your device to a state to which you’d had wished to restore a full backup.
Then, they only back up your apps’ userdata, nothing else, you won’t have a full system backup.
This seems impossible to me. The best solution if you tinker with your device seems to me to always check if what you are going to do won’t stop you from being able to flash the stock OS from fastboot. Such cases about relocking the bootloader on FP4 without checking you were able to unlock it after led to this (a quick search on the forum should help you find it).
What you seem to wish is that EDL works on FP5. Unfortunately, this requires factory files to be leaked, and isn’t even the case for FP4.
TL;DR: I think you can’t in any way do a full backup of the OS on the device. If some tinkering fails, you should just hope you are still able to flash the stock image from fastboot.
Edit: I see @AndreasChris was a little faster for some of the comments, sorry for repeating 