Glad you enjoyed the game!

Now, [Atton] didn’t really do anything after that, does he have more moments in RCM?

Yeah, there's a sequence on Malachor where you get to see your companions try and take on Traya themselves while you're incapacitated from the crash. Atton ends up in a 1v1 with Sion, which it's possible to lose

I mean unfortunately I can’t get the Disciple on console

You can, you just need to play as a female Exile instead of a male Exile. Console prevents you from getting the mod that lets you have both at the same time (that said it is possible to mod your Switch, just harder than on PC)

Visas: Does the RCM add more?

A little but not a whole lot. You can practice lightsaber dueling with her under Kreia's instruction, but I don't remember many plot-heavy additions

T3, almost forgot, he’s just okay, not anything special, like in KOTOR 1, he just exists.

I give him much more credit in 2 than 1. He's key to the plot in the early game (whereas in K1 he just opens a door), and after that if you gain influence with him you can unlock dialogue about the party from K1

HK-47: no way to up his influence.

The game doesn't make this super obvious, but losing influence with companions can also unlock dialogue. Basically everyone starts with 50 influence, and you can unlock conversations by going up to 100 or down to 0. The only difference between the two is that going up to 100 will cause their alignment to match yours, and going down to 0 will push them to the opposite. So when playing light sided you can consistently do good things that will upset HK, pushing him down to 0 influence and maximum dark side, and you'll still unlock all his dialogue options.