I’ve been down this exact route of designing prototypes for production.
I can understand why you want to develop the prototypes by hand yourself - it gives you more control and the part does need to advanced modelling skills.
In my case I worked with a local model maker who drew up CAD from my drawings then vacuum cast the parts. It cost several thousand dollars and when we took it the factory they had to start over and make their own prototype.
So, I would say it is best to work with a factory who will be able to develop it on CAD for you and save lots of time and money.