macOS App Store alternative?

There is no equivalent to the App Store and some functionality of macOS requires you be on the App Store to use it so there’s no competition really.

Historically however MacUpdate has a really good library.

Setapp is an App-Store-like subscription service. They charge you 10USD a month and you can use a ton of apps they have contract with. It does contain a lot of useful apps.

I personally use 9to5mac to check on useful apps and buy them separately on each developer's website.

They are not necessarily alternatives to the App Store, but buying apps separately usually gives the app developers higher privilege than buying them on the Mac App Store (in the example of Commander One).