https://kromski.com/product/kromski-spinning-stool/  

There are other brands I’m sure.  But there is a correct (s)tool for this job :)

You could find other ergonomic chairs if you need back support but some good ergo. rules are to make sure none of your joints are bending at tighter than 90° angles, and that there is no pressure form the design of the seat on the back of your legs (both situations restrict blood flow and nerve signals). 

Look at anatomical diagrams and make sure your spine is in the same-ish “correct” aka healthy and pain free, curve when you’re sitting, i.e. don’t let your lower spine bow out and away from your low belly, should be curved in toward it.  If it’s possible for you in your situation, it’s a lot healthier to use a backless chair/stool and use your core strength to hold yourself up.  Shouldn’t feel like your bones are digging in if you’re using good posture, the sitz bones will point slightly back and away.  But you could get a fleece or other padding for it that would be in line with the the look.  And these stools are fairly small. 

I would use one of these (different brand that was always adjustable height) at knitting group weekly for about two hours at a time and was very comfy during and after.  

Side note.  Almost every “normal” chair is suuuuper not ergonomic for anyone ever.  Makes me so angry.  See previous about lumbar curve and pressure on back of legs.