I'll just share my experience on the journey of shifting better. Mine was a v8/m6 car, but it had some similar symptoms. My 1-2 upshift would grind and/or lockout if rushed. It was ok going down. My 2-3 would also crunch just a touch if really banging through the gears quickly. The car had 16k miles on when I got it, and was doing this. It had a Hurst Billet Pro Two short throw shifter which was pretty sloppy feeling left to right, and noisy.
First thing I did was flush the trans fluid and replaced with Redline D4 ATF. That meets/exceeds factory specifications for a TR6060 - I'm not sure what the V6 cars specify. The fluid change smoothed the trans out quite a bit in day to day driving, but it did nothing to solve for my upshift grinds.
Next, I tried the Ideal Garage master cylinder and dedicated clutch fluid reservoir. It moved the engagement point a little higher on the pedal, and seemed to help some with the upshift grind. Still didn't eliminate it.
Next, I decided I would throw a stock shifter in the car to see what that did for me. My theory was that the 44% throw reduction I was getting with the Hurst shifter was causing my shift speed to outrun my clutch speed. Going to the stock shifter made a tremendous difference. It felt like I could finally drive the car. My 1-2 grind essentially disappeared. I can still introduce it if I go for a full hero maximum effort 1-2 shift, but it's totally fine other than that. The shifter feels nice and weighted, and is dead silent.
For my car, I think the ultimate issue was that the Hurst shifter had very worn bushings - causing an incomplete/weak push from gear to gear. Without sufficient pressure the syncro couldn't do it's job, it so it would grind or lock out.
I'm not sure if that helps, but I thought I'd share my experiences with a similar problem. It was extremely frustrating, making me doubt my own driving at times. The relief and joy I now get driving the car makes all the chasing worth it.