I watched your video. You counted 7 'clicks' when applying the hand brake.
Was that after applying maximum effort ?
Fwiw, if memory serves me right, I had a problem where my 57 Jazz (original calipers and pads) would roll on even a very slight incline with handbrake applied normally (around 4 clicks) due to the cable perhaps stretching over time. Yes, if I was pull the handbrake up further to 6 or more, it would hold the car but that requires far more effort than normal.
I can't remember exactly what I did a few years ago, but after some experimentation, I think the way I resolved it was to adjust the cable until the disc just starts to drag with handbrake set at 1 click (or may be 2, I can't remember). With handbrake disengaged, when I wiggle the cable at the rubber boot at the caliper, there is a bit of slack in the cable - ie. cable is not pulling on the caliper.
Handbrake has been fine ever since for past 4 years. No issues with overheating or dragging rear brakes. On 7 clicks, the handbrake can hold the car on one particular Very steep road nearby. Otherwise, only 3 to 5 clicks required depending on gradient.
I just checked the handbrake operation. 3-4 clicks 'normal' operation. I can only engage 7 clicks at most. There is some further movement of the handbrake lever but 8th click won't engage.
Changing the subject, what make of brake pads did you fit? Did you fit any shims as I couldn't see any on the outward facing brake pad?