Update: I was able to get my first activation! I arrived quite a bit early, and to my disappointment, both of the frequencies I had issued an alert to were in use, so I shifted up, down, etc and called CQ to no avail for probably 3 hours. I was also struggling with a ton of atmospheric noise, and a rainstorm narrowly missed me. Finally decided I should self-spot on the SOTA Spotter app (I was hoping to get my first activation without needing cell signal), and got 5 contacts in a matter of minutes on 20m!
I feel like there should be a SOP for when your alerted frequency happens to be in use (i.e. shift up in 3kHz increments until you get a clear frequency). I'm not sure how practical this would be in use though, as the station I hear that is using the frequency might not be audible to chasers, so they might have no way of knowing I shifted up or down.