"Decals" is a bit of an ambiguous term. They can mean "just" stickers that you actually just stick on your frame (and can remove them without much hassle/anybody noticing). They could also be stenciled decals that you apply on the frame during painting process and be removed at the end to have the name/logo/symbol painted on the frame. Like you would have with the names of all major manufacturers.

I eventually went down the lane of a full custom painting with decals and all. Was good fun messing around with colors and styles. Attached is a mockup I built myself to present my idea to velobuild. I also included the font and the logo as PNG for them to print. Let's see if the final result will look anything like I imagined it

That is amazing! What program did you use? Planning to do this as well.