Useful info! In over 50 years of cycling I'd never had discs, didn't have a clue, but helping my grandson with his bike taught me a few new things! I've fitted hydraulics to his but it's not an ebike so yeah would need research to put hydraulics on my Hunter. Was very simple to move to cable discs as a first step, and the whole upgrade cost under £30 so no biggie to upgrade again.
You can buy new and used hydraulic brakes for the same price. Fitting is identical - basically three screws. After fitting cable disc brakes, you have to do two adjustments, one to get the fixed pad close to the disc and the other to get the cable length right. You don't have to do any of that with hydraulic brakes. Just put them on with the three screws, and they work perfectly. There's nothing to research.
I'll explain the difference.
Hydraulic brakes have two pistons that push each pad onto the disc. As the pressure is the same on each side, they're self-aligning. When the pads wear, the level goes down in the reservoir a bit, so the system is self-adjusting.
Cable disc brakes have a fixed pad and a moving one that's operated by a screw or ball mechanism. The one moving pad has to bend the disc onto the fixed one. As the fixed pad wears, the disk has to be bent further and further to reach it, and the braking efficiency becomes less and less. Also, the pads wear at an angle because of the angled disc. After adjusting the fixed pad to reduce the amount of bend, the angle of the disc when making contact with the disc, no longer matches the pads, so efficiency is not good and the wear rate is high. The system is flawed in its concept. Once you try hydraulic brakes, there's no way back.