Hello @Kathleen1121 ,
You've mentioned that you have check-out instructions. Consider sending them to the guest a day before check-out, reminding them of the check-out time and procedure. Also, consider mentioning that if they want a late check-out, they should check with you in advance, as it depends on availability. You can also add that there might be an additional charge, but this part depends on your hosting style.
Consider automating the message so you don't have to think about it.
In my experience, guests start to wonder about check-out early in the afternoon the day before, so this is when my automated message is sent.
When guests take a long time to leave, you can use a method I read in the community a while ago (sorry, I don't remember who wrote it). Explain that you have to start cleaning because you have another check-in the same day, and then turn on the vacuum cleaner. This will rush the guest to leave.
Although I have never had to use it, it sounds (literally) like a good idea to me.