Good post and on the money.
You need to be less available.
You are far too available and your text is too open ended.
Put a time limit on everything and then when that time passes there is no issue.
"Hi A, would X like to come here on Saturday? If you could let me know by Y time, no problem if it doesn't suit you?"
If they don't get back until after that time, just say another time.
When you are firm with your boundaries and stick to them, people get the message.
My children had expensive lessons and whilst there was some flexibility, only with notice.
I certainly wouldn't be paying for lessons simply because a parent didn't give a firm time.
We would simply politely decline.
Anyone taking 4 days to reply would simply be treated with a No thanks.
So any invitation you issue needs to ha e a reasonable time limit on it.
Any vague invitation you receive likewise needs a reasonable time sensitive firm time, otherwise it's a No thanks.
You have years of this ahead of you if you don't get a grip.
All 4 of my children had lovely primary classes of 30 children but they retained no more than 4 when they left.