Until Steam have
Civilization VI listed for Linux as well, I won't be buying the game. So effectively, I probably will end up buying the game like a week or so after release.
As for Steam, my experience is that if you own the game, you own the game for all platforms. Even platforms it will be ported to later.
I have multiple anecdotes regarding this, but recently I bought a small game called
Turmoil when it was only available on Windows, because A) it was on sale and B) the developers had promised macOS and Linux ports in the future. The moment it was released for Linux, it suddenly appeared in my games list (which I restrict to games with Linux ports).
I assume the same will be the case for
Civilization VI. If you pre-order it now, the moment it is available for macOS and Linux, you should be able to install it without problem.
I've only encountered
one game on Steam where Windows and OS X ports were sold separately. This was shortly after the release of the Steam client on OS X, and I am sure it was a demand from the publisher rather than Valve, since no other game had this separation on Steam.
And since
Civilization VI will be distributed
through Steam, you can safely purchase it on any other site, as any other store will only provide you with a Steam key for the game. And since the game on Steam will be available for all platforms (once the ports become available), those keys will also grant you access
to those platforms.
Just to confirm, am I correct that if I were to set-up an account and login with Steam on my current computer, download and play the game. Then if I get a new computer (or use my wifes computer), I could just login with my existing account, and also download and play the game without having to re-purchase?
Correct.