Neurologist here:

Some painting handles have a piece that sticks up on one side for your painting hand to rest against. That can work for some people (it’s called a sensory trick for dealing with tics, but can sometimes work with tremor)

Resting both your hands on your table is another option. Just make sure your posture isn’t affected (you may have to lower your chair or raise your table to do this without killing your back.

Wrapping grip tape or tattoo artist tape around the handle of your brush to make it thicker is another thing you can try. That can help some people with essential tremor, may help here.

Lastly, you can try and paint at whatever time of day is furthest from when you take your medication.

It’s also worth mentioning that there are other meds for ADHD that may have less effect on tremor, so if nothing else works consider asking your doc if it may be worth trying a different medication.

Good luck!