Tbh I think 1500km combat radius may be a bit low for 6th gen.
Current 5th gen fighters can do 1500km already, and those which cannot may be able to achieve it with variable cycle engine upgrades if considered for it.
I think a combat radius of 2000-2500km may be necessary for 6th gen fighters.
In what configuration?
Typically in modern civilian turbofan engines, you can achieve about a 15% improvement in fuel consumption from 1 generation to the next.
F-22 has a combat radius of 460 miles with just internal fuel according to this airforcemag article. Maybe it will be a little more if it flies all subsonic.
F-35A has a combat radius of about 1100 km according to online sources, but I'd imagine that's in an A2A profile.
I think it would actually be quite impressive if J-20 can achieve 1500 km with purely internal fuel. It'd probably have to be flying high subsonic and carrying just its standard AAMs.
I think comfortably achieving 1500 km combat radius with more than just A2A missiles is probably a pretty good goal for 6th gen aircraft. Maybe they can hit 1800 km with just A2a missiles.