Yes, leave.
I know you say that you dont want to cut your nose off, but in my opinion you'd only be doing so IF it was that the people were actually OK to work for. But they're not, they're making you very unhappy.
Something has to give and from what you have said it doesnt sound like the people who pay you are going to budge, therefore its up to you to put up with it or leave. I know this sounds unkind, but i mean it as it is is written; i'm not trying to be nasty.
What you want (by the sound of it) is to do the hours which suit you, at the rate of pay you are happy with, and without complaints. And why shouldnt you. But the reality is that so far its only the hours which you are getting; the remainder is making you very unhappy.
I have to say that one of the reasons i went self employed was to avoid overbearing bosses like the ones you are describing. There is no way i would stand for it from a client, i would walk and keep walking. It sounds to me like you are in search of a magic answer as to how you can make these people come round to your way of thinking. But in truth there is no answer. You can't even say that the fact that they like you is enough for them to want to change their attitude, as you give the impression that they neither like nor respect you and the work you do.
When you talked about unions, what did you hope they would do? Was it to get them to make your client change their thinking, or was it to make a financial claim, etc? I do really have to question why it is that you seem so keen to keep doing business with them
