I'd focus 100K of that flying on whoever gives you the best perks at that level, including award travel. Are you looking at leaving EXP behind, then, and just falling back on Platinum?

For instance, UA at 100K requires you buy up to an upgradable fare to use systemwides on international travel, while AA does not. If you want to fly places AA does not go, and have a chance at upgrades, you should look at 1K at UA. Just keep in mind that some of those upgradable fares can run $700 higher (like SFO-SIN), and there's no guarantee, or refund, if you don't clear. Not everything is grand over at UA.

I'm leaving UA this year (did my last paid flight with them last weekend to IAD) because I don't like the changes to mid-tier and lifetime status there.

How about award redemptions at DL? I've seen occasional complaints that reasonably priced award travel can be a challenge there? Have you checked the Delta forum to see if that is still the case? No sense in building up miles you can't use.