KDE does have transparency built in. Go to settings and search for Translucent (something like that) and it should come up. It’s under window rules or something like that. Not at my computer or I’d tell you exactly. You can set it individually for inactive windows, menus, etc.
As far as the panel, you may have to modify the contrast value of the theme’s plasmarc file to something like 0.2 or 0.3.
Edit: also found this old post on it. But I know you can do some of this topic.