Okay, I have some suggestions on the second barrel, contact Wilson Combat and see if they have a 300 Hamm'r barrel for the Rem 700. it will be a improvement over the 300 blackout, or so I am told. The 350 Legend would be a huge improvement over all of the rest of those cartridges you listed.
I was going to suggest before you restricted me to a 223/0.374" bolt face some other calibers than above. I was going to say build a heavy twist 243 Win, heavy twist 6mm Rem, 7mm-08, heavy twist 260 Rem, or a 257 Roberts. all of these I have taught girls as young as 7 years old to shoot comfortably.
My friend down the road (28 miles down the road) has a 6.5 Grendel he loves to shoot. Come to think about one more suggestion that probably will not be made.. 6.5X55 a wonderful low recoil cartridge.
short receiver and small bolt face.. kind of restricts the possibilities.. you could get a larger faced bolt and open up the 0.473" case heads. this would be best. If you go with a detachable mag you can switch bolts, barrels and mags and have the most versatility.