There's nothing illegal about using sigpatches especially if you use them for own homebrew forwarders. NRO2NSP doesn't use any copyrighted stuff to build the NSPs either.
As u said u will get banned though.
Theoretically u could use RetroArch on your online console if you boot it the official way without getting banned (but you never know).
However I've been using the forwarder NSP for ages too on my emuMMC and never had any RAM issues.
Seeing what just happened to TX, I think RA / Atmos devs are wise to keep a degree of separation from tools enabling piracy for current systems.