Thanks for the response and clarification. I know that the thread I was quoting wasn't about extroot, but I found anyone metioned it there aswell, specifically here, that's why I was pointing to that one.
You are right about the space, I might be a bit paranoid about running out of space because it happened to me multiple times with the TP-Link Archer C7 (but that one had only 16MiB of flash) and I thought since it's already there and I play with it quite a lot, why not have all the space available, but I agree that only reason to have it as root would be for opkg packages, otherwise it could be mounted as separate partition.
I am not sure how this procedure with patches, backporting and pull requests works for OpenWrt git, so I thought I would just post it here in case somebody wanted to do the same (even if out of curiosity or having some other purpose for that)