I made a small tool for exactly this annoyance on Plasma:
You want a different icon for one application, but you don’t want to switch your entire icon theme, and you don’t want to dig through .desktop files every time.
kAppIcon lets you:
Map a custom image, another app’s icon, or a single glyph from any installed icon theme (WhiteSur, Tela, Breeze, Papirus, etc.) to one launcher
Do it completely user-level (no root)
Batch-apply the same icon to multiple apps
Undo the last change
Create/edit icons with a simple pixel editor
See all your overrides and reset them cleanly
It’s built specifically with Plasma in mind, but works on other freedesktop desktops too.
Repo + install instructions:
https://github.com/rayman1972/kappicon
Also available on the AUR (kappicon / kappicon-git).
Feedback welcome. Bugs can be reported via 'issues' in the project.