I am actually using and testing KDE/Plasma on a touchscreen device (an 2in1 laptop).
I use KDE Neon and I used 1-click to open which interesting but can be frustrating in particular with a touchscreen and there is my experience and some propositions for improvement.
Basicaly 1-click to open is efficient on touchscreen (and with mouse) but the target to select (selection mark) is too small and often I miss it leading to missing my selection and/or open the file which then I have to close the programs, so lot of frustration. On the other hand 2-click to open seems we could do better. The problem is more important with touchsceen UI but it could also happen using mouse.
My idea, which also come from the UX on android file manager (ghost commander) is to provide a 3rd choice for select/open. We can see it differently but a simple way is to take the 1-click to open + selection mark and inverse it or see it as a split icon with double action: one area to select (bigger) and an other to open. On the Android file manager the item is selected when taping the name of the item and open on the icon.
This approche also looks a little bit like a proposition made for icons in the systray.
split icons
The UI I think of is selection by taping/clicking to the major part of the icon of the file/dir and open with an open mark on the corner. The selection coulde be additive like in selection mode. So the better of both world.
An other approch is selection mode. Selection mode is interresting but cumbersome: needing a long press or taping a button in the toolbar for such a comon/repetitive action. A possibility, could be a persistent selection mode with a button to open (single or multiple files) but I found the interface in selection mode to clutter with the 2 bands maybe with more integration (with the toolbar for exemple ) or even the possibility to get rid of those green band, personnaly I already have cut/copy/paste and copy/cut to the other pan in a toolbar (which is on the side).
I was thinking of that for some time and I wonder if it could be implemented and if it interrest others.
Sorry for the long post english not being my main language I have difficulty making more synthetic.