I have decided against installing KDE as a replacement on my mainpc for now because it is so different from XFCE that a huge learning curve and setting everything up from scratch is needed should I reinstall with it. It does also seem to be fairly bloated with some of the installations on the VMs being very slow with tons of files to install. Using it in a VM is a useful workaround if running it natively under XFCE is difficult but there are XFCE settings that control the drag and drop and playing with these seems to be one avenue I can explore for now.
Subsequent testing in VMs has also confirmed the issues are not resolved by using KDE instead of XFCE so I plan to stick with XFCE and the KDE experiment is officially over for now.
I am also rearranging the screens with the second screen for mediapc, which is on the left, swapping places with the only screen for win10pc, which is on the right. This will let me use win10pc’s own Microsoft multimedia keyboard on its own and thus have all four keyboards easy to use, where it sits currently on the right it is too far away to use easily, but I can use the spare keyboard slide on the left (that is actually normally used for my drinking cup) for the MS keyboard being easily within reach. So then using the Win10pc as a fourth computer alongside the other three without switching keyboards becomes much more straightforward. When not in use the keyboard will still be stored on the right as I don’t use this computer very much with the MS keyboard, I generally use it with one of the switchable keyboards, but there have been times when I want to use all four computers at the same time and having Win10pc’s keyboard on the left and screen on the right isn’t very user friendly. This also dispenses finally with the ideas about having an extra sliding shelf of some form or another on the right, I won’t be doing any such thing now, what is on the right are the two KVMs and the scanner and these will remain unchanged.