But that's not "discouraged by the system UX" and what better way to get people to use f-droid than offering incentives to use it? Like your favorite app having better features when installed from there?
Sorry, but if you do not believe getting security warnings, having to go into the settings and mangle with configuration, just to be able to install a store downloaded from a website, sometimes go through additional configuration depending on your phone's modifications (looking at you Xiaomi) to then install the actual program you wanted is not "discouraged by the system UX", despite the real world experiences of companies who have tried even simpler things and failed, I'm not sure we can agree on what's viable or not.
The amount of time it took you to type this response is longer than it takes to enable third party sources in settings. Most people do it at some point anyway, such as when downloading an app they want from it's website. There's no mangle about it, it literally pops up with "allow this app to install applications?" and takes you there directly.
If someone wants f-droid they'll do it. The problem is most people don't know what f-droid is. It has nothing to do with UX friction of installing it, there is virtually none.