[Closed] Help with weird gear setup problem

I wonder if anyone can help with a weird issue I'm having that I've never experienced before.
I'm struggling to setup the rear shifting. This is a brand new bike build I just put together:
X0 9 speed gripshift
11-34 PG-990 9 speed cassette
X9 medium cage mech - this year's model so 10 speed (but i thought actuation was the same as 9 speed?).
Cable routing is under the downtube, full length outer apart from a short section under the chainstay.

Now, what seems to be happening is I get a full 9 gears travel at the mech from only 8 clicks at the shifter, even if I start with a little slack in the cable when in the 11t sprocket.
I set it up so it will just shift from the 11t to the 8th sprocket (i.e most slack i can have in the cable), it goes 9-8-7-6 OK, 6 is getting a little rattly, then going to 5 bumps it up to 4, and by the time I'm at the largest sprocket i still have a click left, and depending on how it is set i either can't get that last click, or it is really tight.

I've messed around endlessly with cable tension, tried the b-tension, triple checked the set screw stops, but no luck. What's going on? Are the new 10 speed mechs incompatible with 9 speed? I had an almost identical setup on my last bike for years with no issues.

Any help appreciated,
Paul