Testing stuff in Age of Empires II HD

I am trying to get a foot back into AoE 2 after not having played it for 15 years. I would like to get a gut feeling for how well different units do against one another and how powerful upgrades are and so on. Maybe even practice some beginner level micro. As such, I would like some quick and consistent way to generate units and buildings, select or deselect different upgrades for each of the two armies, and then pit them against one another.

What is the easiest framework for doing something like this? I am at the moment running AoE2 HD on Steam.

This is impossible unless you create a custom Scenario for each of the things you are describing. A sandbox map doesn't exist. Your best bet is to play against an easy AI or battle a friend and have them build units against you.