Switch from AR15 to AR10 308 would be far more versatile and in the right build fairly trim in the weight department. PSA 12.5” pistol would make a dandy truck gun.
I have been contemplating this exact thing for a couple weeks now. I've come to the conclusion to just get an AR10 instead of converting to a different round. It's more expensive but I'd rather have 2 functional guns than 1 gun and a different upper.
From the list of options you are considering, I would go with the 6.8. You will get more range out of it. I would also throw out the 6.5 Grendel for consideration as well. I built one a few years ago and it is a tack driver. I can typically get .5 MOA groups out of it, but if I take my time, I can get it down to .25 MOA. It shoots better than any of my bolt guns which has led to a great deal of frustration in my bolt guns.
As has been mentioned the Grendel was not listed. I own all of the ones you mentioned plus. And my go to is the Grendel. I have hunted everything from black bear to pronghorn with it. It has never missed a step.Attachment 307070 Plus I own a AR10 in 308 and 6.5 Creedmoore. My Grendel is still my go to.
I have AR-15s in 5.56X45mm, an AR-15 in .50 Beowulf, an AR-10 in 7.62X51mm, and an M1A in 7.62X51mm. They cover everything I want in semi-auto rifles.
6.5 Grendel for all around use.
224 Val for long distance shooting (less drop and drift at distance) with lighter recoil.
The 224 has almost twice the retained energy at 600yrds over the 300Blk round. It can beat the 6.5mm Grendel in wind drift and trajectory, but the Grendel throws a larger diameter bullet
and a heavier payload with more retained energy. So, if precision at distance is your deciding factor the 224, if ft/lbs at distance than the 6.5.
Also, the .224 Valkyrie will essentially run through your standard AR receivers. All you will need is a .224 specific barrel and bolt. Not the plus-sized receivers typically needed for your .308 Winchester or 6.5 Creedmoor.
Of the calibers you've listed as an additional upper to your current AR I'd have to go with the 6.8 ........ ammo is going to run you between $ .60 & 1.00 a round though.
5.56mm will get the job done. Id much rather have a 5.56mm AR than any of those other calibers. Especially if youre limited by budget the ONLY other option is 7.62x39, and with the options of quality 5.56mm or 7.62x39, why would you go with 7.62x39? 5.56mm is just a better round. Theres a reason that every country that uses AKs that isnt a third world dump has gone to the AK74 pattern guns and the higher velocity 5.45x39mm, because for intermediate rounds, a smaller, flatter shooting, higher velocity projectile does more damage. On top of that, has anyone ever seen a 7.62x39 AR that runs like a top? Having worked at a gun range for a year, I know that I certainly havent seen one that works well enough that Id ever rely on it for bedside duty.
If you want to varmint, buy something like a 50gr ballistic tip bullet. If you wanted to hunt hogs, go with something like a heavy OTM or solid copper bullet.
Just roll with 5.56mm. If you have to have something different for the sake of being different, so be it, but just know that youre giving something up to do so.
I like the 6.8 and the AR platform makes it such a handy deer rifle. Low recoil and accurate, I liked the 6.8 so much I barreled a blot gun in it too.
My hunting is all 200 yard or less, itís perfect.
The "prepper" who hides inside me says that one's rifle(s) and pistol(s) should use ammunition which is always readily available, keeping in mind that eventually, as civilization dies, you may have to loot some retail outlet to replenish your supply.
Military ammunition, and its civilian counterparts, will always be the most likely to be available.
Granted, this is away from OP's needs/wants, but I thought I'd post this, after some of the comments. I too debated on AR/7.62X39. After checking into options, I went with the PSA KS-47 Gen II. I love this thing! I know it's long, but this guy IS THROUGH! (a NO-NONSENSE view on firearms review/information) Actually found this, after my purchase, & he justifies what I already knew. Jump to 24:25 to see his thoughts on the PSA.