When I was a lad I severed my right index finger clean through the first joint with a penknife – the brand new blade cut right between the bones and sliced through both tendons, it was hanging on by a thread of skin. I was very lucky to have it reattached by a visiting specialist plastic surgeon who was teaching in the area, and he put it back together instead of just chopping it off, which is what most docs would have done at the time. It took over 6 months to heal properly and years for the nerves to heal fully, but it did eventually start to work ok.
It’s still wonky as **** and quite weak, and if I catch it on anything it hurts like hell. But with modern disc setups (I use Shimano XTs mostly) I can use it to control my front brake perfectly on its own. Keep trying, OP.
Oh and you probably want to run your brake levers flatter than you currently do, about 15-20 degrees from horizontal. Makes a huge difference, that does.