I have enough spend to hit top-tier on pretty much any program (except M&M HON), so I'd like to now just pick a program that gives me the best chances of international upgrades. Any thoughts?
Define "international upgrades". If you are referring to op-ups, just remember that it's a US-only idea that people will be upgraded just to fill seats upfront.
If you are talking about paying for upgrades, UA remains your best bet since their upgrade instruments can be used on LH as well. You seem to fly in premium cabins a lot, so it's clearly not a matter of status. I'm not sure what the situation is for UA, but LH lets you upgrade on their metal also from lower economy classes. It's probably no use to you though.