Google Play Store review might actually be worse now. It's certainly slower. It's great that you can sideload at least on Android although making money off a sideloaded Android app is very challenging to say the least.
That said, I am left wondering why more major software publishers don't have the option to sideload Android apps. (It was a while since I looked into it, since I mostly use open source apps these days, but it seemed as though most companies were switching to a Google Play Store only route a few years back even though they had the brand recognition to bypass Google's store altogether. The only thing I can think of are support costs, since few people would know how to enable the ability to load apps from other sources.)
Is that really the case? Why wouldn't you be able to? The FCM APIs aren't gated by install method; I've successfully used them while sideloading my own self-written, unpublished apps.