Google just seems to have glommed everything into FCM. In the FCM console, push-notifications are really treated as a user-engagement/advertising thing. Throttling and spam prevention makes sense for that.
Maybe Android has something like the iOS VOIP mode that doesn't involve annoying the user with scary battery notifications, but I couldn't come up with anything at the time.
In fact, it didn't matter at all once the initial notification was visible whether follow-on notifications were displayed. Possibly the fact that I was locally dropping the notifications (because the user didn't need them), the system counted those as "notification that weren't interacted with, so I'm going to slow you down at the server."?
I have no idea, and no way to find out, since FCM is pretty opaque at this point.