Wanted to say the same as the-entropyst. I may would do it depending on size that is available on the window.
- There are situations when all 3 elements (main categories, sub-categories and settings) are visible. In this case I would just show all 3 elements.
- If you shrink it horizontal, the main category disappears. In this case I would show the tree view to give more information and allow for faster movement between categories.
- If you shrink it horizontal further, currently nothing happen. But if you want to make it phone friendly, you could collapse it another time and just show settings or the tree view.
- If you shrink it vertical (like phone on landscape), it depends if you want to keep categories and settings visible at the same time, then tree view, or if you want to fold pages, than showing categories and sub-categories together and settings alone.
- [Edit: It would also be a valid thing to think about max amount of entries, so that we do not need to scroll through 10 or more sub-menus or to make such things a setting itself → 0 is tree disabled, -1 is always enabled, 5 means with 5 or less sub-categories it shows the tree view, with 6 or more it just becomes an own column aka disabled.]
Why do I speak about phones? I think most KDE-software in general (except desktop Plasma) could be fully convergent with some minor tweaks without compromising desktop experience. And if we speak here about how to handle sub-categories, it is very close to speak about convergence of settings-application. As far as I can see, beside of folding in one way or another, there are just some minor adjustments required as minimum display size (which is bigger than needed anyway) and maybe some specific setting-pages as WLAN & Networks (horizontal scrolling is usable, but not optimal).