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.
Or maybe not, idk