Hearing loss runs in my family. When it's time for me, I plan to roll my own. I suspect that if you get rid of just one requirement, it becomes easy. That requirement is to stuff the whole thing in your ear. If the electronics can be in a separate box (or in a smartphone as mentioned), then analog, signal processing, and battery power all become trivial.
https://photos.app.goo.gl/4lOwfrVJ3bfRWkEm1
(Stereo mics and amp with two-channel parametric EQ on each side. About 6" wide and runs on two 9v batteries, if you're curious!)
He took it to a Bose showroom to compare it to their Hearphones. All (Bose people included) agreed that his amp and EQ was superior. Cost for parts is ~$200. He's had a lot of fun building them for himself and his friends.
One set of keywords that seems to turn up some of the limited info out there is "cochlear body worn processor".
Current material refers to 'bodyworn' for babies and toddlers who tend to lose BTEs. You get the same BTE, but it's strapped more firmly to the body.