This is what tends to give most players trouble against Rash. The most important thing to remember is that when Rash tongues in, it is generally not his turn. Any normal that’s 8 frames or faster (cr.MP is the button HIsako uses) that covers the space he’s in will beat any of his options. Important to note that if his tongue trajectory makes him cross you up, you’ll generally just have to hold the MK that follows because your ant-air will come out wrong side. For dealing with wrecking ball, you want to block the ball itself and then do your cr.HP or other anti-air. Wrecking ball is armored and generally difficult to stuff, so you’re usually better off blocking it and getting your punish after.
To your point, there really isn’t a way to stop Rash from getting on top of you if that’s he wants to do. Your power in the MU is to be able to punish those approaches after he bullies his way into your face. A lot of the standard Rash pressure (tongue approaches, wrecking balls for days) is punishable so long as you know where the gaps are.
As Code says, you just have to hold that. All of the characters you play have similar 50/50 mixups that aren’t really reactable and that the opponent just has to deal with - KI is a game of strong offense across the board.
As you say, Rash pretty much dunks on Kan-Ra as I understand it. @nSxTyme2Dul may still be able to give you some pointers in the MU though. @SonicDolphin117 may be able to give you advice with Arbiter, and @LetalisVenator can probably give you some help with Raam (light emergence is probably the biggest thing here though, as it should punish all his reckless air approach options). Gargos’ st.HK can fulfill that same role for him, though I’m not really sure how the rest of the fight shakes out.
And finally, I did make a video a while back talking about dealing with Rash as Hisako. It won’t all be applicable, but hopefully some of the stuff (particularly the way I challenge his tongue approaches) can be helpful to you.