I never pre-order, there is no benefit.
Early access is misleading, there are games which are “released” and would barely count as early access and vice-versa, so I just treat them equally.
The criteria for me is that based on reviews or some gameplay footage it seems like I can get £1/hour worth of enjoyment out of it. I tend to look for how many hours do people have when they leave reviews and how many have they played since, rather than just what they say. If I’m unsure if I’ll like it and there is not enough videos or reviews to give me certainty, i may take a risk on £10 and below games depending on how bored I am at the time.
I’m pretty sure the dlc thing is already possible. Guild wars 2 at least works this way, you can buy the game/dlcs either via steam or via their own store and then you can install and run the game either via steam or via their own launcher (although IIRC the steam way still has the launcher).
It’s probably more of a case of steam providing a convenient way for developers to not need their own account system, so rather than them creating their own solution that integrates with steam and other sources, they just straight up use Steam’s way.
To be honest I’d love it if they forced a way for steam and other shops to allow migrating your games between them, so I could take all the free games from epic but never use it. Currently my compromise is to just never use it and skip the free games.