The setup that I adopted 15 years ago and still use today is the "bar desk". It's a standup desk that's positioned in such a way that my elbows rest naturally when I'm standing.

But instead of a fancy mechanism to make the desk go up and down, I have a saddle stool. When sitting on it, my head is at the same level as standing, and my spine is straight. (Key point is monitor position.)

What I like about that is I can swap between sitting to standing in a few seconds without even thinking about it and without waiting for the desk motor to go up or down. This was originally a poor man's standup desk because I didn't have the budget for a motorized desk; now it's a choice.