It would be nice to have (or extend) a <meta ...> tag to address the address bar translucency. Did you submit a feedback for it?
In the mean time, I guess you can use something along the lines of placing a white div on top, out of bounds, hidden behind the address bar.
https://kyj7l.csb.app
Note that the "place something white behind" doesn't work when the scroll position is small in modulus and you have Dark Mode enabled (I'm assuming that that happens because a small fraction of black is used to initially generate the blur effect on the address bar in that case, maybe you can use a black div when Dark Mode is turned on, adjusting the theme-color accordingly?). Furthermore, I used a scroll event listener to remove the white div when scrolling up and an event listener can be quite heavy CPU wise. I would very much appreciate a proper way to disable the default blur.
Tested on macOS 11.6, Safari 15.0 (16612.1.29.41.4, 16612).