It sounds like your chair needs to be a bit lower.
For keyboard work, you want a fairly high chair, so that your elbows are just above the height of the desk, so your hands hang above the keyboard, and you want your screen up, so that you can sit straight. But this really isn't what is right for painting or modelling. But most of us have our desks setup that way - partly because most desks are now sold for computer setups.
Try lowering your chair so your elbows are just below the desk, then you can rest your forearms on the desk - which helps brace things. If your chair won't move try putting a box in front of you to rest your forearms on.
I tried a friends kneeling chair many years ago - it confused me. The idea of them is that they stop you hunching over, as its really hard to do so - but you may just end up too high for modelling activities.
Sitting still for any length of time isn't great. Getting up and walking around is good, even if your posture is good.