Zach-
Welcome to the forums, and to CityStrides.
Don’t feel bad for joining, it is great to have more people hanging around. 
If Strava is your best way to get activities in, you are in luck - your recent activities are already prioritized over your historic import as that is the way the sync queue is set up.
Even with that system prioritization, your new activities will take a bit to show up. Being a supporter helps with the time before you see it on CS even more.
There is a new Pause Feature available, but it is still a bit unclear to me how it works with the already-queued activities (both recent and historic.)
I know once things are current for you, it will be possible to pause your account, then upause it before you do a run you want imported and then re-pause it after to do the selective uploading you talk about.