Would this work the same way if I have a different OS (I have Lineage, I had MM)? I know I already said I did, but I just find it hard to believe it would be that simple considering how risky and complicated everything else was. Also, what's "dd", I've played around in TWRP so, I'm probably just not remembering what that is.The arb you see in SALT would not have been increased but another "hidden" one. But h812 was a good choice so yes you are lucky.
You can do that:
Wipe cache (not dalvik) in TWRP.
Using fastboot and flash: boot, system and userdata. Then see what happens. Actually it should work. If fastboot does not allow flashing userdata there is a way doing that in TWRP as well with dd.
There's no reason for these OS's to be this complicated, I've clean installed windows 2-3 dozen times, why can't a smart phone be similarThe reason you got lucky was probably due to the 'Assert' line in the updater script in the rom zip file. That line checks that your model is flashing the correct rom version for it. It will throw up error 7 if you flash other than an H812 rom. If you tried to flash an H811 (that is allowed on the H812), you would in all likelihood receive the same error - in that case you would have to remove the 'Assert line' in the rom zip file in order to proceed with the flash, all else assumed equal.