Welcome to the forum.
You have asked a lot of (variable) questions but it would help if you could provide a picture (or drawing) of about what you have in mind.
It saves everybody time, including yourself, since you might get less accurate advise without providing the necessary details.
Fwiw, I've machined many injection molds (from billet) that produced hundreds of reject parts before we "got it right".
Shrinkage, vents, gating, etc. each required changing the mold.
Sometimes we had to up the melt temp of the material by 50f to get a good fill. Problem with that is then the whole mold becomes too small (by a few thou) because of material shrinkage.
Bottom line: unless its very difficult to machine, your only reasonable choice is machining.
Fwiw: rigid pvc (eg plumbing pipe) machines well and is relatively inexpensive but it is just one of hundreds of material types available.
Post a pic and what you want it to do. Will it be exposed to sunlight? If so, can it be painted? Oils or other chemical exposure affect the material selection.