Where do you go from here?
Very good question.
First I’m sorry you are constantly stuck in this situation. You deserve better than this. I’m going to suggest some professional counseling just for you. Six years of an on again off again relationship that has multiple instances of lying and cheating and disrespect is not healthy.
You describe the relationship as "him being your best friend" but also "you put more effort into the relationship" and "you don’t want to start over" or "lose him and his family".
It sounds like you are either very happy or very unhappy depending upon the day and his behavior. You seem like you are living in an emotional roller coaster as his GF.
Long story short, he has an addiction to sending photos to people. Has it gone further than that? I’m not sure. But it doesn’t appear that he values certain ideals such as honesty, monogamy, respect etc. Adductions are very difficult behaviors to stop without professional help.
Your married life w/ him will be exactly your life now. Marriage will not change nor stop this pattern.
Again I strongly urge you to get some professional counseling. It will help you understand so many things about your BF and help you make some decisions about your future.
I hope this helps you. Notice I’m not giving you advice or suggestions on what the lying cheating BF should be doing. IMO there is no point to that as he has issues of his own that need to be addressed.