Well, but you said that you don’t want to maintain it and you don’t have time. That’s basically the catch with all of this. You maybe should think outside the box, because all the things you mentioned in this thread go wildly against what you yourself require from a system you install for your relative.
Doing an arch install with KDE will be incredibly annoying in the long run and will give you A LOT of work to make it usable for an inexperienced user.
I have an own life and a job to do, my time is limited. So I am f*cked anyway.
That was my thinking, too, and I installed Linux Mint. Since then no problems with constant support requests from my mother. It just works for her tasks. And if your relative wants to just use a text editor, browser, light photo editing and email program, this is probably the best course of action. So maybe you should ask your mother what she wants to do with her PC and act accordingly to that.