Foxydroid is a really nice and imo much faster F-Droid frontend https://f-droid.org/en/packages/nya.kitsunyan.foxydroid/

(not affiliated just a big fan)

I think that these demands should require the built-in app store to support an interoperable feed to other sources. I think that they do but not clearly enough.

I doubt Google would interpret it that way, but that would be the fair way to do it. Make it so that the information is interoperable rather than just not actively disallowing it. Then it can stop being a monopoly.