Their payment system doesn't even support multiple cards saved at the same time, but having a checkout and using a CDN is ecommerce basics that any hobbyist can setup. That's a very low bar for UX.

The Epic Games store doesn't have libraries nor a 'locate' feature, so if you want to move a game you have to go to the existing game folder in Windows Explorer (no Mac or Linux by the way[0]), move it to a temporary directory, start the install in the new directory, 'cancel install', move your game files to the directory the launcher just created, and then 'resume' the install. Only then can you move and have the launcher recognize the new location without having to redownload the entire game.

0: https://trello.com/b/GXLc34hk/epic-games-store-roadmap (it's not even in 'future development' which includes other long-requested features like a shopping cart from 2019)

This is certainly not the common case. Look at other games, Doom Eternal, Hollow Knight, Arma 3, Divinity: Original Sins 2, ... They don't offer that.

Look at the automatic update settings. There is no option to not update a game at all such that you can keep playing the current version. At any setting it will still check for updates when you launch the game (for the first them since starting the Steam client).