I only found your introduction after I wrote my first response. I have a friend who uses what you call a rollator. I also have reduced mobility myself, though not as significant as you. I have seen for myself the challenges that rollator users face when travelling by train. I hope that you can find a way to make your trip with the minimum of stress and without wearing yourself out.
Do you have a plan for getting to and from Kings Cross that you are happy with, or do you have outstanding questions about the journeys? I try to avoid travelling between Kings Cross and Waterloo. Unfortunately some of my pre-pandemic options for avoiding that are not available at the moment.
And have you been to Kings Cross or any other major London terminal station before? Everything is very spread out at Kings Cross/St Pancras.
One other thing I'd suggest is to avoid Kings Cross during school holidays because of the Harry Potter fans.