I have replaced the original 512GB SSD with a 2Tb one and reimaged the drive. Do I need to keep the original drive for warranty purposes in case I need to return the device, or I can re-purpose it or sell it out?
I would keep it. Even if they accept the warranty, you have a good chance of not getting your SSD back. Are you really going to sell it for enough to be worth it?
You can probably use it somewhere else, though. I don’t think they care if it has the right image, and if they do you could re-install it, or dump enough games to clone the drive back.
I’d ask Valve.
I just slotted my original m.2 into one of the additional slots on my motherboard. It’s not a ton of space (only 256GB), but it’s free and I had the space. And this way I still have it if I need it.