Might as well post it here. I see this quite a bit, people asking for some more roundness in plasma ui. Here's what you could do.
1) use another application style instead of breeze. Lightly for example.https://github.com/Luwx/Lightly It has a bunch of settings, roundness being one of them. Fair warning: If you want a transparent toolbar, that seems to have issues since they implemented that wallpaper/custom highlight stuff in plasma.
2) use another window decoration. Klassy https://github.com/paulmcauley/klassy has some more advanced options.
3) use a plasma theme that has roundness in it. You'll find some in the kde store (or make one). In that screenshot you'll see what I mean with that radio widget.
4) use a more rounded icon theme. If you want to stick to what extras the breeze default icon theme has to offer, you could do the rounded version https://store.kde.org/p/1313336. There is also a chameleon version https://www.pling.com/p/1608766/. If you want to make your own color schemes, there is a breeze rounded script. Can't seem to find a link to that one anymore or I made it myself ( can't remember). Anywho, here's an example of some ol' ubuntu style I did ( you get the point).https://imgur.com/a/KtKvEzd. So, in order to do that, you'll need to have the BreezeRC icons installed and adjust this script https://github.com/RaitaroH/BreezeDarkIconColors.
5)the inner roundness of a kvantum theme is defined by the svg's. If you want roundness ( like, for example, the highlight), you'll need to fiddle with the svg's themselves

So, there are options available if you want a more round look. These were just a few. https://imgur.com/a/k8fgl6n