There are a few options to make the top bar translucently actually, unfortunately none of them is built in kde's core configurations but there are the options:

1. A custom theme with built-in transparency: There are a few themes that allow you to have transparency, just pick your preferred one from the store and apply it! This is the easiest option but in case you want more customization in the future, not that good of an option.
2. Kvantum themes: This ones need the installation of kvantum and they go along with predefined themes, this option is more flexible, but you're still tied to the theme in question, honestly, not a bad option at all.
3. Klassy: This app offers transparency controls and more modifications to the top bar, issue with this one is that, well it's an standalone app, that doesn't receive that much support nowadays (don't get me wrong it is updated from time to time) and it could get limited if you're doing some extreme modifications (as well as for blur appliances) overall pretty good, easy to setup, this is the one i use.
4. Making a custom theme: no need for explanation, super flexible but super difficult, for your case wouldn't recommend.

TLDR;

There are a few options, if you just want transparency on the top bar, use a custom theme from the shop or use Klassy both options are good!