Suggestions for braking with severe osteoarthritis in both hands
A coaster brake for the rear comes to mind as a main brake. Refer Sheldon Brown ), however for safety you need more than one brake.  A high quality MTB disc brake is designed for low force i.e. single  finger for normal use, and high end come with a vast array of lever adjustments so you can get the levers close to the bars to reduce the reach needed.
With Hydraulic disc brakes only need to provide hydraulic pressure from a hose to make them work, it may be possible to fabricate something that allows you to generate the required hydraulic pressure from something that suits you hand ability, you may find a specialist who already has such a device.