Have you tried hydraulic disk brakes? They use a lot less force for braking, because most of the force of a cable-brake goes into stretching the cable. Also the power of the disk brake counts a lot: the more powerful it is, the less pressure is required to activate it.
Also, note that reclining two-wheelers and trikes are worth having a go on, they give you many options of changing the brake lever so that you can push the brake with your palm/elbow/leg.
It's possible to re-purpose a grip shift throttle/gears if they use a cable, and also check [old-fashioned gear shifters].
I'd say add a disk brake and arrange a push-lever on some kind of touring handlebars, else try a trike and add third brake on a push-lever and a pad somewhere near your elbow. perhaps even try a third brake near your knee when it's raised and have a sideways knee pad on the frame.
Take up african-trance dancing!!! There are lots of dancing videos (search "choreography + alice/julie/fatima", and with Covid-19 I find them very useful. Africans go into a trance when they dance, and that is true of every sport: When your body gets into a rythm, it's possible to keep going for 30 minute or an hour, especially if you put in some upbeat dancing music.
Honestly, for Covid-19, I have learnt to dance into a trance to keep fit, sprint with your arms while hopping around in circles, slash the air like a tiger round and round and up and down and left to right while also hopping left and right and around, pretend that you are 4 years old and jumping up and down for your birthday, go frenetic, and you can sprint 100 meters in 1 minute, or keep it chilled and get your heart to 120 for 30 minutes, it's mostly a factor of finding a rythm and going into a trance, I also have weights in my sports room lol. choreography videos give you 1000'ds of complex body moves to copy, and sweating hyperactive young people to emulate, and you can always play your own music in the background.
Also check your swimming center for exercise groups and walk up mountains :)