hey thats kind of neat.
Nice. Essentially, your method sounds like that of Magisk but done manually . Question : Does root persist ? Can you, for example, install busybox binary, reboot, open terminal and run busybox? what triggers the manager to create a the modified boot.img ? could you create 1 just prior to reboot? then what ? if u got root still problem solved but if its the original img. start the process over lol ? wait .....temp root with magisk then flash SuperSu for perminent root, wouldnt pass safetynet but you wouldnt have to flash the img every boot ? im fascinated.....w/o magisk to automate process its hard to see it being useful in terms of systemless root that persist but it might be used as a means to attain ole school root. i think u could use flashfire for the process . if the devices doesnt power cycle it should keep root, flashfire boots to the bootloader which would still be the modded img if it isnt allowed to go the secondary program loader (idk but im working under the theory that the bl, doesnt reload the img on said partition from hotboot)