Beyond those two specific characters: in general, Ctrl + Shift + U followed by the code point does seem to work consistently in Plasma if you enable “iBus Wayland” as a virtual keyboard. (Without that, it only works in GTK apps.) However, I found that enabling the iBus Wayland VK gave unwanted side effects, so I ended up using compose sequences for my needs.
I don’t think you can get what you need using compose though. Tilde is ok (compose key, then space, then -), but there doesn’t seem to be a compose sequence that gives you a backtick (aka grave) if you don’t already have that character on the keyboard. Reference: GtkComposeTable - Community Help Wiki