you can’t, actually

You can sponsor a promotion; sales on a bunch of games - but it's not "Brought to you by the cool refreshing taste of Pepsi" it's like "Berlin Game Developers".

You can't. Check the very bottom of this page: https://partner.steamgames.com/doc/marketing/visibility

Steam as DRM is basically the best case scenario. It is opt-in for developers, meaning there is no DRM at all for some games on Steam. You can download it, back it up, run it without Steam. The games that do use Steam's DRM are trivial to "crack" by replacing a DLL in game folder with a stub (you can find open source ones on GitHub). If Steam had no DRM I think publishers would lean more on other options which are worse for customers.