I sadly, and I don’t say this lightly, have to agree.
When first using KDE, I was totally baffled by the amount of themes you can download.
But I quickly learned, that most of them are either more than 5 years old, look ugly/ incoherent, or cause this blurriness issue OP mentioned.
For my taste, Breeze (or to a further extend, Lightly) looks phenomenal by default, especially since Plasma 6. I only change the color scheme a bit (usually just adaptive colors, based on wallpaper), but that’s it.