Just don't imagine that a three week trip is in any way comparable to a 6 month trip, or that mileage in Europe (including Eastern Europe) is comparable to mileage in Latin America.
You've got enough time and money. It's just that you don't have an excess of either, which means you'll constantly be making choices about where to go and how comfortable to be. That's fine, as long as you're both on the same page about it.
I took 4 months from Seattle area to Ushuaia. I hurried for portions, didn't hang out much, and missed a lot. Then I took another 8 months riding home again; I hurried for portions, didn't hang out much, and still missed a lot. It's all about priorities and making clear choices, mixed with a certain amount of luck and going with the flow. The one piece which is probably not worth risking is your relationship with your S.O. This part, too, is going to be different on a longer trip.
Good luck!
Mark
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