Not exactly what you may be looking for, but one of the first things I set up in Linux is the “Compose” key. Sun keyboards in the 90’s had a dedicated Compose key, and you can enable the functionality still. I usually set it to Right Alt.
The Compose key is kind of like an extended shift key, so ‘Compose’ + “c” + “/” for example will give you “¢”.
The key combinations and characters can be edited in a config file (can’t remember off the top of my head).
Not as versatile and an “Emoji picker”, but allows quick insertion of Unicode glyphs into text. Useful for ¢£€¥™×° type characters.
That’s exactly why I always enable the Compose key. It’s the fastest and easiest way to just type a variety of Unicode glyphs. The key combinations trend to be intuitive as well.
There’s a good chance the default config file will have a pretty decent selection. Although I have edited the config in the past, I haven’t done it under KDE. The KDE article on setting up the compose key seems to say that KDE uses a different config file anyways.
Turning on the Compose key is pretty straightforward as I recall, just another setting under Keyboard settings. Finding that config file is still useful if you can’t guess the right combo for your desired glyph.
Very useful for using character common in math and science.