I really just don’t see how this could ever work how you are proposing.
I couldn’t do my job part time as that would mean I’m doing it half assed - even if all I had was one project on my plate I need my 40 minimum hours a week to get it through the product development cycle on time. I’m just merely one cog in the machine and if I’m only working at half speed then that means everyone else is waiting on me to deliver.
The only way it work is if you didn’t have your own projects but came in as an extra set of hands. For example you assist a lead designer on making a model, doing some sketching, doing renderings etc but I’d never really take too much ownership. In reality I’d say that is what interns as they have those required skills whilst shadowing/learning from a fully employed designer.