Hello Ian and welcome to the forum.
You are on the right track to use graphics rather than geometry. It is much more efficient on the system.
Decals can be made with an alpha channel using the image tool in Creo (or older Pro|E versions). I would suggest an application that gives you all the symbols you need for the project. There has to be a font that has all these. If nothing else where you can capture all the symbols with the Character Map. Maybe a calculator app? Or even PTC's free Mathcad Prime 2.0 app? Something where you can easily screen-cap each symbol at the right size and scale. Once you've collected the images, you can apply an alpha channel with the Image tool so only the symbol remains once you apply it to the key surface.
It is a bit of work, and once you are done, I hope you can share it here. A symbol library like that could be very useful. Maybe you can make a Document on the process if you have time.
If you do find these characters in a TTF, you could also load this into the Creo font folder and actually call up the characters in a sketch. You will need to know how to access the extended font set but it can be done this way if that is preferred.