If your desire is to build a mobile app, you don't really have a lot of options.
An app store is really many things. It's client software for installing apps, it's a service for verifying apps, it's a hosting service for apps, it's a payment processor.
If you want to replace any piece of it you currently have to replace the whole thing, and do so all at once. Even if you create a good alternative app store, it has few apps in it, which matters if it's a separate client/interface than the main one. And you have to replicate every piece of it well for it to be any good.
So what's needed is better modularity. You don't have an "Amazon app store app", you just have a "software installer" app that shows all apps from enabled providers and then lists individual apps as "verified by Google", "verified by Amazon", etc. in the same software. When you buy one you get a list of enabled payment processors and choose "pay with Google" or "pay with Paypal" or "pay with Visa" etc. (the list being all the ones that the app developer accepts).
Then you can put your app in Google Play and let them do everything, or you can host it yourself and pay a verification service that charges a one time fee rather than a percentage and accept five different payment services but only the ones that charge less than a 5% commission, and either way it's not a significantly different experience for the user.
And whether or not Google would join in on something like that, at least Amazon/Samsung/F-Droid could do it with each other (and anyone else who shows up).
And if you are using Google's identity system—in what way are you not just back to publishing on the Play Store? Through the identity system (and associated developer-reputation system) they can control who can or cannot publish on your app store. So it's really just their app store. It's your UX, but it's their guiding hand, just as if Google were on your board of directors.
Really, there's no point in them letting you have your own app store which relies on parts of their infrastructure, since they can tell that any implementation their legal dept would okay is a lose-lose that no dev would ever be interested in taking part in. To detach any liability for the results from them, you have to do it all the app-store stuff yourself. Which is exactly what mobile OEMs do.
Amazon doesn't require you to use their payment system with your customers to host your app on AWS. Paypal doesn't require you to host your apps with them to use their payment system. Mozilla doesn't try to stop you from using Firefox, or the Firefox-based Tor browser, to visit arbitrary "dark web" sites, and Debian doesn't try to stop you from adding arbitrary third party repositories to apt. Let's Encrypt doesn't even require you to use their own software to verify your site. This concept of "you can't just sell someone a screw driver because what if they kill someone and you were liable" has no basis.
Google could refuse your business for an individual component if you tried to use that component from them, but then you could just get that component from someone else. It would still be an advantage to be able to use one service for hosting and another for payments, even if zero of those services are Google.
And the problem with Google Play is the weak competition. If it was easier to use something else then they would lose more business from making poor refusal decisions (harming their reputation), by charging a high percentage, etc. Then they would either have to improve or lose their status as the dominant player. Reduced barriers, more competition.
The proposal was to host your own app store which doesn't rely on their infrastructure.
Consider disabling the apps; if the manufacturer doesn't allow disabling the app through Settings, you can always do it via adb. When you disable the app, the system will ignore it and behave as if it wouldn't be there. You can't get any additional benefit from the deleting, you won't gain any additional free space, as no app can write to /system partition for the above reason anyway.
It depends on what you mean by "side channel". You can do a PWA instead, or use tools like Test Flight (up to 10,000 users), or leverage the Apple Developer Enterprise Program, etc.
Epic Games. https://techcrunch.com/2018/09/07/fortnite-hits-15-million-i...
Do you mean being able to click a link on the app author's website and press "allow" makes you an elite sort of geek?