2019 kx450 rear brake is too strong

Try to get a stiffer return spring for it. It will slow down the travel thus making it feel weaker. You can look on mcmaster carr for springs. Another workaround is to use your clutch to prevent stalling

Cut a groove in the brake pads. Less surface area.

Bleed your brakes. Flush the whole system with new fluid. You may have moisture or air in the line. When your brakes heat up it causes the moisture or air to expand. This can cause the brakes to be overly aggressive.

Weaker is easy: shave some material off the pads. If you grind down the back edge of each pad at a 45, it even makes rear wheel swaps faster (Baja-style trick where the rotor slides in easier). You could also test different brake pads and varying compounds, but the problem is that the rotor has too much leverage now at 250mm.

Good info here: https://motocrossactionmag.com/mxa-race-test-the-real-test-of-the-2019-…

*New fluid is always a good idea on new bikes as well, though it may not help with your current situation. Old school front brake trick by leaving a zip-tie on it overnight with increasing pressure also seems to help the Kawi's more so than other brands...

Also, adjust your clevis so that the contact point is farther away from your master cylinder plunger. It won't actually decrease the braking power, but it will feel like it because you need to depress the pedal farther for the same braking action. Sometimes that's all it takes. It sounds stupid, but try it.

Weird, I didn't notice anything different in rear brake performance between my 16 and 19. I haven't measured it yet, but I bet they changed the master cyl and caliper piston diameter to compensate for the bigger brake diameter.

If anything, my 19 is much harder to stall than my 16 was because it is mapped much better. I used to stall that 16 out all the time!