Having played through the PS5 version on diff 4, and a relative of mine claiming he can beat it on PC on diff 5, I came to this page to see what the Renegade Legend WR was, and saw to my astoundment that seemingly every single run had been played on diff 1 where Ratchet/Rivet supposedly can't even take damage or recoil from enemies, making the runs almost unrecognisable as being Rift Apart.

I'd understand if it'd be much to keep track of if there were 5 difficulty categories, but I greatly wish for at least a few Renegade Legend (diff 5) subcategories to be added, even if it'd just be, for example, NG any% and NG+ any%?

This has been brought up once or twice in the past and hasn't been met favourably by runners of the game. Already, anyone can run on higher difficulties if they want (the leaderboards don't need to dictate what runners choose to do), but since the game's release 2 and a half years ago we've seen zero interest from anyone when it comes to properly routing or running a higher-difficulty category. The consensus among runners is that the added health and damage is more annoying than challenging (and it does not materially change anything about how the run is performed other than tacking on a few minutes of fighting, considering runners are good enough at combat anyway to avoid taking unnecessary hits). Historically, this has also been true for every other RaC game with difficulty options. The community greatly prefers the emphasis on performing tech and movement quickly and keeping a consistent flow to the execution of the run rather than being bogged down by combat. As it stands, there doesn't seem to be a reason to add a difficulty subcategory unless there's a sudden influx of runners who're willing to play it - even then, it's likely it might end up on the category extensions instead.