If you look around, everyone make it complicated. It isn't.
* Settings > Hardware > Keyboard
Select the correct keyboard modal, then on the Layouts tab add a new one for your language, layout, and select the 'type 7' variant.
Move it to the top of the list so it's default.
Enable nothing in the 3rd Advanced tab
* Reboot - you should find the volume and power keys work
* Settings > Shortcuts > Standard
Go through the dedicated keys like "Undo" on the keyboard, search for their short cut and add an "Alternate" by clicking the little arrow on the row. Tick "custom" then click the tool icon next to it, and press the hardware key. Something appropiete should appear in the box.
* OK and apply everything
* System Settings > Shortcuts > Custom
For things like "Stop" where it makes sense to have it do Ctrl-C, add a new group (such as "Sun") , then add a new global shortcut to "send keyboard input". Setup a trigger as with undo above, and use "Ctrl+C" to send a "fake" key press.
If you want the giant "Help" key to really be "Esc" you can do that here too.
* Reboot - copy and paste etc should now work
(Kubuntu 20.04 but should work with KDE on any modern Linux / Ubuntu)