Miniature painting requires intense concentration and many hours in a stooped position, either sitting hunched over a desk or with the painting propped up on your knees. So you have a choice - one day, either your back goes or your knees go. Which will you sacrifice?
For me it's likely my knees (they are ugly anyway). My back is important. Well, my whole body is actually important, but if I had to choose...
I wish I didn't have to choose. Someone should start a miniature painting yoga class to correct our posture.
Ha ha, so far it's been my back AND my knees... AND my neck! My covid at-home living room studio is not exactly ergonomic...
...but it is beautiful! And akin to the old miniature painting workshops of old, which were floor-based. So you're right to prop yourself up with yoga cushions and the like, and have your painting upright too. I love seeing people's studios or workspaces! Maybe could make a section here to have a peek around... also, eye exercises are another thing I'm interested in. The main thing is to rest your eyes occasionally, close them, get good sleep etc. And brushes - I see many people keep them with the hair side upright in jars, which may be good while painting but not so much for long-term storage (they gather dust), so I store mine flat in brush holder where they can also breathe. Chinese brush holders are designed to hook the brushes so they are stored with hair down so they preserve their points. I love your carpet, I love your floor, I love your sofa. Your empty wall just needs one of your paintings now... ;-)