I have been using Global Theme “Breeze” or “Breeze Dark” with “Maia Transparent” – from “Colors & Themes >> Plasma Style >> Get New” – and it has been working fine for years.
It is more “readable” if Blur is enabled in “Window Management >> Desktop Effects”.
There are other “Global Themes”, and / or “Plasma Styles”, and / or “Window Decorations” which give you transparency – but many of them are not available for Plasma 6.
So, I could keep “Transparent Oxygen” (Window Decorations) in my previously installed distros – but I could not get it in a new installed KDE Neon with Plasma 6.
Indeed, the lone “Panel Translucent / Opaque” has been frustrating for me, since the first time I have tried it, +/-2016.