I am about to finish a two year long course in CAD but despite the fact that I can now use design software well they didn't actually teach me how to design anything. Pick one that not only appeals to you in work type but also the benefit of tuition assistance. I agree, you don't need to have an engineering degree to be a great mechanical designer. In essence, you perform pretty much similar function to an engineer, but do not have all the responsibility. In other words, you design the part and the engineer checks to make sure it won't fail and kill somebody.
Most degreed engineers aspire to be good designers.
A person with as much shop experience as you would make a great designer. Let someone else do all the math while you take the beginning engineering courses to see if you really want to be an engineer. Dom, I hope I can give you a useful reply. I work with, and help make hiring decisions for, both engineers and designers. Knowledge of math through trig, engineering drawings, principles of electronics, hydraulics, and manufacturing processes.