Yea, this fixed the problem. Great!

I built it and installed the cursors now. They are also all correctly listed in the cursortheme KCM (kcmshell5 cursortheme) and I was able to select them all individually. I've seen only two problems :

• On auto resolution it selects size 32px for my 1.5 scaled display, although it should be 36px. But this is probably in KWin, still we shouldn't forget about it (if you are interested in how it works grep "themeSize" in KWin's source and "cursorSize" in Plasma Desktop's source).
• When using auto resolution the mouse movement seemed to me in fact somewhat more sluggish when before with only 3 sizes, but this might be an illusion. Did you try your patch on a slower machine?