It's a fun idea but from a design perspective there are some flaws.
- Having a slow that allows you to guarantee even more damage (because of Kata's point-and-click E and R) is flawed. For comparison, see Zed's E which also slows but his only other damage ability is a skillshot and not point-and-click I like the interaction of kill then slow though, but I am not sure if it's balanced in general.
- Having significant, near-guaranteed sustain on an assassin is flawed. This is by all glass cannons' design.
- Onhits on Kata's R are a point of build expression: Kata can choose whether her damage will come from raw stats (AP and Magic Pen) or through Onhit effects. It does not significantly change Kata's R as a spell in any way, therefore removing them is both unnecessary and removes build expression. I will note that it is fine for the most part if you want to consider her as a pure AP raw damage assassin.
- Buffing base AD scaling on R is not the correct way to compensate for removing the ability's Onhit effects.