I ran a 6" wheel with a harder LR and it was rolling pretty good.... until the RR hub starting sliding in. Then it became a handful and overworked the RF tire.
I have ground a notch in the spindle, but for you application that may be over kill.
You said your RR slid in, so that RR was wrong from jump.
Sounds as if you aren't running a kart that is intended for asphalt ie aluminum axle, 5deg. LF spindle L block combo and the seat a little more left than usual.
And the RF was wearing because of the harder LR you put on, you took grip from there and forced the RF to do the work, the RR sliding in didn't do it.
In fact the RR should never slide in, as I posted on 99% of the karts I work on if the RR moves in ⅛ in. it's rubbing the frame or cassette