I've got an e-bike and have always struggled with adjusting the rear disc brakes as I can't reach the inner adjuster, without removing the whole brake caliper. Guess this is caused by the fact the rear wheel has the electrical motor which takes up too much space. See photo below. I've worked around it by taking the brake out, trying to guess how much to adjust, and then trying to put it back in; aligning with other bolts to adjust horizontally. This is a very time consuming process to ensure it doesn't rub AND it has enough tension.
I'm trying to follow this video which seems very easy, if you can reach the inner adjuster. Any tips? Perhaps would work with a flexible allen key if they exist? There's only about 1cm of space.