I find, at least till now, Epic client pretty clean, it still miss a lot of "basic" features, but when I close it I don't see running stuff left around, the same for Steam and UBISOFT Connect.
Also if I manually launch a game, downloaded from the Epic client but that doesn't require it to run, I just can do that and the client, with all its components stay closed. (Or at least I don't see nothing relatable running on my system.)
From this point of view, right now, it's GOG Galaxy the worst one, because it leaves services around even when shut down, and those services will be automatically started from the games even when launched directly without the client running... there is no option at all to remove or disable those services.
If you manually stop them, and then launch a game (directly from the game executable), those will be started.
If you remove them, and open Galaxy, those will be created again, and will stay there.
So, if you want a game to not use them, the only choice is a batch that disables or removes them before it is started...