Hi. Went early last May with my adult son. The last time I was there was 2000 and I was shocked by the change — mainly in terms of volume of people.
First, I suggest you arrive at the processing area by 9:30, 9:45 latest. As you approach there will be Vatican employees on foot who will direct you to the correct line. Because you have reservations, you shouldn’t have to wait. The ticket you have allows you to enter and follow a ton of signs to get in line for the actual ticket. The whole process takes a long time and the tour will not wait for you.
Be aware that there is an absolute crush of people. If any one of you has claustrophobia, it’s rough. Photos are difficult because you will in all seriousness be back to front and side to side with other people. It’s still all amazing to see, and to understand the vast wealth and treasure in the Vatican. Note that the Sistine Chapel allows no photography and you must be silence.
At the end, there is a way to enter to the basilica through a shortcut — if they make that possible for you, TAKE THE OPTION rather than choosing to explore the galleries further on your own, or you may have to wait a long time in a new line.
The Basilica is GORGEOUS and you will have FAR more breathing room, there :)