Might just be me, but I see 300BLK or x39 as being minor variations of a 5 56 gun while .308 is a more significant step. Sorta like engine options in an F150 vs making the jump to a 3/4 ton.

You gotta decide for yourself how far outside of your current box you want to go. What is the intended use that is driving you to 30 cal? Is it is just to have something different? What do you expect the gun to do better than a 5.56? You seem not to have defined answers to these questions.

In my case I went from multiple 5 56 ARs, added .308, then 9mm and .22lr.. I have yet to add 300BLK and I have AKs in the x39 flavors but not ARs. I also have multiple other guns in a spectrum of loadings. My ammo logistics are way more complicated than yours.

Though I have not followed your path of stocking limited cartridge types I do find merit to that philosophy. Were I in your shoes I would consider adding cartridges to the mix only if you feel the need to fill a specific role. I seriously consider myself to have an overly complicated "logistical chain". I stock 5 pistol calibers, 7 rifle calibers, plus .22lr and 12 Gauge. Your way is better.