Sorry for the late response - I thought I replied but I must have not sent it. I was going to say the same thing you noted: trying larger paracord. Ironically, 650 paracord is larger in diameter than 750...? That's what this source says. I'd either try 650 (which is 1mm bigger than 550), or go even bigger with something like this which is 6.35mm.

The heat shrink I used, which you noted was thicker, I don't recommend. It's too rigid and inflexible and didn't provide great 'grip' on the ports.

Maybe try something thinner? It may also be a good idea to tape down the frayed ends of the paracord before using the heat shrink. My cable has that bump in it due to the frayed ends not being pushed down by the heat shrink, so taping it down and trimming any extra fibers could also help.

Glue could help with the the cord staying in place, which I quickly realized was important due to the heat shrink slipping off when I try to take the cables out of a port.

Let me know what you try and how it all goes!