Another suggestion for the
X-series, especially the earlier games in the series (Beyond the Frontier etc).
Yet even
X:Rebirth, the Complete Edition, feels grand compared the vast majority of today's space games.
Elite Dangerous. If you don't mind Steam or XBox (it's coming to the PS4 as well), this might suit you. It's easier to get into than an X-series game and offers an even bigger sense of scale.
Evochron Mercenary, I haven't played it myself yet but it's on my to-get list. Similar to Elite Dangerous and the X-series. Available on Steam as well as
directly from the developer/publisher. Whether it's actually DRM-free or not I haven't checked up.
There are not that many games around that will meet all your criteria though.