So in my opinion, I'd go Saum parent in the 65 and 25, you can get away with prc in the 6mm. My goal is 3000-3150 fps with high bc expanding bullets, NOT using crack powders like N570 and RL26. The extra velocity you get with crack powders is just a lil icing on the cake. In my 22in brux 25prc, 3100 is very near psi with H1000 using 134 eldm. In my 19in 6prc H1000 shoots will with 112s at 3150, dtacs were near psi at 3100, couldn't get enough N570 for psi at 3130(61.5gr), 57.8 gr rl26 is 3200 with a dtac, but it's not far off psi either, shadow ejector marks flat primers. I also have a 24in 6saum, long ass freebore, and it's fast, like 3400 with a dtac and N570. But I feel that's unnecessary velocity.
I think if you go equal barrel lengths on all, say 20" the 65 Saum, 25 Saum and 6prc will all operate in that 3100fps realm with temp stable cool burning powders.