Since you have the 6.5 Creedmoor in an AR-10, then you won't really be benefitted to do a 7mm-08 or a 243win. The 338Fed makes logical sense for "something different".

Are 200grn pills advantageous for whitetails? Nope. Do they work just fine? Yup. The only real consequence is recoil, for which a heavy AR-10 in any 308 based case is not really too extreme.

In your shoes, I would do what makes the most logical sense, with a secondary influence of what makes most financial sense. As a matter of speaking, that is to say this: I would buy as many lowers as you need, or you can afford. I have several AR's, but if I look at things objectively from a "how many lowers do I need" standpoint, I only really can justify 4 lowers - a benchrest lower and a telescoping carbine lower for me, and the same for my wife. If you only hunt with your AR-10, and you only hunt/shoot alone or with someone that will have their own rifle, then you only really need one lower. Alternatively, if spending a little extra cash on things you don't really need doesn't hurt your feelings, then it doesn't really hurt to have an extra lower that you just "want" but don't really "need".

Right now I have several AR-15 builds in line at my bench, one of which will be my own. It will be one lower with 3 uppers. I also have different bolt action rifles that accept different barrels (takes FAR longer to change than an AR!!), and a Contender with different barrels. Many people want complete rifles, some people are OK with sharing parts.

One other advantage, beyond simple cost savings, of having only one lower is that you get better familiarized with the trigger and stock fit.