I think that sounds pretty good. I understand your frustration - Sammy will sometimes only go once a day, skipping her nighttime one. If she does, she's up at 5:30 ready to go outside
but trust me it gets easier once they are trained. I used to do like you - sit outside for 15 minutes and wouldn't let her back in or she'd run and go in the closet. But now she's trained and she will just ring her bell whenever she is ready. It's ALL worth it once they are trained, trust me. Mine were both fully trained around 18 months. They were good with supervision by 1 year.
Usually walking them, rather than just standing in the yard, will make them go. I do this if we are going out for the evening. I feed them and then walk them and they will both go within a block or two. It's nice to know they are empty if I'm going to be gone for a while.