1. The "post office address" is usually just "city, state zip", not a street address. Some people like to add "Hold for cross-country cyclist" on the line after the name. This is unnecessary, but it might add some amusement to the small-town postmaster.
2. I've never seen nor used the 9999 suffix.
3. Every town that has a post office accepts general delivery. But not all post offices do. So if a town has multiple post offices, all the general delivery will go to one of them (it's not always obvious which one). For this reason, I try not to use general delivery in towns with more than one post office. You could end up having to go to more than one to find your package. Also, as you note, many very small post offices have very few manned hours. The USPS website shows the hours of each post office, so be sure you know what they are. Note also that every town that has a zip code doesn't necessarily have a post office. As I said, the USPS website identifies all post offices. Only look at real "post offices", not "self-service kiosks", nor "approved postal providers".
4. As mentioned above, you don't get to pick which post office your general delivery package goes to. A big city like Albany would not be my choice because of this.
BTW, there are many threads in this forum about General Delivery. If you search, you'll get lots of other information and tips.
Buffalo Prairie, IL has a real post office, but only has very limited hours (see below). But you can send them General Delivery if you want and are sure you're going to be there during their open hours.
18401 206TH ST W
BUFFALO PRAIRIE, IL 61237-9000
Phone 309-537-3169
Pickup Services Hours
Mon-Fri 8:00am - 10:00am
Sat 8:00am - 9:15am
Sun Closed
Interesting side story. The post office in Dalbo, MN is very small, similar to the one in Buffalo Prairie. But they know that almost all general delivery packages are for touring cyclists. They also know that almost all touring cyclists stop at the Adventure Bicycle Bunkhouse a couple of miles out of town. So when they get a general delivery package, they give it to Donn Olson at the Adventure Bicycle Bunkhouse and put a sign on the post office door saying where it is. Donn puts it on the table in the bunkhouse.