Very good summary of a pertinent issue and great ways to try and mitigate it. My sister was a fashion designer and here the days were various festivals as well as weddings when she would be swamped with work. She worked non stop for many years and finally it was not making any sense for her and she was losing her passion. Finally she just stopped and went into another field. I can see in your field how difficult it is to say no, cause your repeat customers are your most valuable asset and they are the ones who you cannot say no to casually. I am very impressed that though you have learnt to cope very early in your career. This is a very good indication that you will stay ahead of the stress in the long run.
Cheers from a fellow #dreemerforlife