TL;DR: If you have an HIDPI display, use wayland.
HIDPI and KDE... I was sure it was an issue of the past. I got KDE Neon to test.. and it was a nightmare to have "just the minimum" of working UI. And after 1 hours of fixing .. A small comment on reddit : "Wayland fix that". Wait.. Neon is not on wayland by default ? Ah! okay then. **Switch to wayland** : Now all my "patches" start to "overwork" (everything is too big) and after I removed them, now everything look fine.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko