So do you still have other kids when she picks up? I would hand over baby, say had a good/bad day, see you tomorrow, I have to go and turn around and leave. She won't keep talking if you are not there to engage in the conversation. If she follows you back into the home then you really have an issue and you need to be totally clear with her. If she has left the home you should not open it back up for her or does she just come in?

My parents are allowed to just come in to pick up their child but once they leave any who need to come back (forgot something) always knock.

I had 1 mom who was similar to yours but not to that extreme. Our daughters were best friends and we would often chat and some days it would be a long time. On the days we were outside at pickup and I wasn't in the mood to talk I would just tell my daughter it was time to go in as we are in a hurry today. If we were downstairs and she was the last one I would start turning out lights.

You need to stop this behavior and stop it now. Yes you will come off as rude to her but there is no other way. She isn't getting it and you are enabling her