I went with an overkill approach at first (as I often do :-) and bought some expensive nicely designed "active chair" / stool that was adjustable high enough so that I could lean on it even when using my desk as a standing desk. It was interesting, but not a game changer at all for me. I don't use standing desks now at all.

But what I have now is this:

https://www.amazon.com/dp/B002FL3LY4

Just don't assemble the backrest at first. If sitting up straight, I just lean wrists on my keyboard wristpad and part of forearms on the desk, no armrests needed either.

Edit: I still use my height-adjustable standing desk, but now it's value is that I could adjust it for the perfect height for my sitting-up-straight position (so no chair armrests needed) and it's been fixed at that height for the last 7 years...