The Workspace->Desktop Behaviour->Desktop Effects->Appearance->Translucency->General Translucency Settings->Menus slider changes the opaqueness of menu text, not backgrounds!

This makes fully transparent themes difficult to work with because the menu text is confused with the text beneath the menu, and there is no apparent way to make the background more opaque to prevent this.

This can be seen with fully transparent themes like Agik-100-transparent in applications such as Konsole or Kwrite if the menu falls over text in this or another window.

I am using Kwrite 18.12.2 on Plasma 5.15.2 on PCLinuxOS, if that makes any difference.

I'm pretty sure those translucency settings apply to the whole window - text and background. You just may not be noticing it when using a translucent background because the background is already (partially/mostly) translucent.

For proper translucency support you need to use a desktop and widget theme which supports translucency so they can take care of applying the translucency to the background elements while leaving the foreground elements (text, buttons, etc) opaque.
Desktop themes (under Workspace Theme) provide their own transparency settings (including the use of blurring and the degree of transparency) which must be changed in the theme itself in order to customize, while application widget themes (under Application Style) may provide a dialog which allows customizing the transparency.
The default Breeze application style doesn't support transparency, however there are other application styles you can install which do, including:
* QtCurve (should be in the repos) - provides a configuration dialog which can be used to tweak the translucency settings among other things. Many themes available on the KDE store.
* Kvantum - various themes available on the KDE store which provide their own translucency settings.
* StyleProject - experimental but highly customizable style.

For transparency with Breeze (or other application styles which don't directly support it), there are look-alike themes for QtCurve and Kvantum on the KDE store, or you may try the Windows Blur script on the KDE store.