I’m not sure this is KDE Panel you’re talking about, but if it is, you can configure border radius in Panel Colorizer:

Damn, turns out it’s Blur effect strength that is killing transparency. Found a suggestion to turn it way down and now I actually see transparency working in window decorations. I never noticed it’s even possible to configure blur further in desktop effects. Thought it’s just an on/off flag.

I tried even more of manual installing, for example I gave a try installing aurorae windows decorations alone, and transparency never works. Turns out, transparency might be totally broken with Wayland on KDE / Plasma 6, at least there are many issues searchable on this topic and even AIs seem to have learned from somewhere that “In Plasma 6 on Wayland, transparency effects may not work as expected due to various bugs and issues, especially with certain themes and drivers. Users have reported that transparency works better in X11, and some have found that specific settings or themes can help achieve the desired effect.”

I also tried Maximized Window Gap (fixed metadata and manually specified margins), but it doesn’t seem to do anything on current KDE/Plasma.

Yes, it is not handler by Kvantum, which means if you install a Kvantum theme, it will not make your panel match that theme, so you still have to do that yourself. I found out about “Panel Colorizer” widget for KDE Panel, which looks very deep and promising, I’ll try and see, maybe I can come up with something good by combining multiple approaches.

As I mentioned in OP, I also tried Kvantum, but it didn’t work well because what’s the point of using it if can’t restyle taskbar. One of the most important things to customize for me personally. Ideally would like it semi-transparent blurred/glassy, like a shader or something.

I tried both through discover and manually by downloading. For example the most popular theme in the KDE Store called Sweet KDE, didn’t have Dolphin and some other things transparent+blurred like showed in screenshots. The only thing which has some transparency is console, but its transparency looks kinda bad, because it’s not blurred. I also tried re-enabling blur in effects system configuration, but it doesn’t affect it.