It’s a very interesting observation overall that both high contrast and low contrast can be good for accessibility.
We could consider our current symbolic icons a part of a “low contrast” mode, and give them some subtle color for a new “default contrast” mode. This could could be done via internal CSS, so it just turns on and off, no need to swap out a different icon theme. A high contrast mode would probably involve a whole different icon theme and dispense with the symbolic icons entirely in favor of full-color ones — which would of course need to be created and maintained, and that adds some risk. In the meantime, the new default contrast icons with a little but of color would have to suffice.
Considering how many people might have some similar needs as you have (might be as high as 1% IMO as already 0.7% are registered in Germany to have ME/CFS), it might even make sense for KDE to provide a “Low Contrast Colour Scheme for People With Neurological Conditions”. Of course for this we 1. need the idea, 2. a plan what to do, 3. agreement to the plan, 4. someone to work on this. Not sure though if there is some one-size-fits-many solution there in the first place.
Anyways, I am mostly rambling, but there is theoretically a way forward here.
The widget in 6.2 is known to be a little weird especially on different aspect ratios. I rewrote a lot of it in 6.3, and hopefully it’ll work much better.
In regards to icons, maybe it would make sense to split symbolic icons and full-size/normal icons when it comes to theming? Maybe an extra tab in the icon settings for the symbolic ones where one can then choose between different themes and additionally how the icons should be coloured. I imagine that a symbolic icons pack would provide at least a monochrome variant which gets its colour from the plasma colour theme but can be made darker or brighter with a slider, and optionally a colorful variant. Also the icon size could be set independently, maybe?
The benefits in terms of accessibility would be that you can more directly change only the part that’s bothering you (and have more options for that too), instead of having to find an icon pack that has both normal icons and symbolic icons that fit you. And people could create icon packs that only contain the symbolic icons, to fit specific needs, without those packs being tethered to the specific full-size icons that they are set to inherit.
A side benefit is that all those broken (and abandoned) plasma 5 era (or earlier) icon themes would suddenly be useable again.