Hello Ellasmum, i don't usually reply to such long debates as this but I really feel for you and think you need a big hug.
I think you have a lot on your plate and that you're trying to make everyone happy including yourself and I don't see a problem in that.
Your question wasn't about how you live your life and how good a mother you are but about sunday timings - so my answer to that one is perhaps your ex collecting them at 10:00 and dropping them off at 4 or 5pm would be better so that they can wind down after their day, have dinner, bath, cuddles and bed for school the next day.
I do get what the other posters here are trying to say and it is food for thought however from what you have said your dp sounds like a really nice person and he makes you all happy. You sound as though you have been through a really hard time with exP and need to be loved and deserve to be happy - a happy mum surely is beneficial to her children. So far you are doing your best which is great. You are even taking a parental course, can't ask more than that. Keep things on good terms with ex, you are juggling a lot of balls in the air at the moment so keep strong and stay focused.
On another note, just from my experience though - i have a step family but i have always made time just one on one with my dd, be it just taking her to the movies, park, playing games or going swimming, it keeps us connected and gives us quality time without her sb or ss or sd. We are all happy together as a family also but that quality time is our special time.
Good luck and i hope it all works out beautifully, you sound intelligent enough to know what you're doing 