I am not trying to circumvent their donation system
What gave you the impression that itch is exclusive? Circumventing would be to sell people stuff that is on itch but processing it elsewhere. Whyever someone would want to even do this, there is not reason.
You might be circumventing certain laws, especially tax laws, if you accept "donations" but give out a product later. That is not what a donation is.
ordering it in an unfinished state
Uhm. Status in development and payable would be quite normal here. But I guess you worry about those steam key integration. But if you plan to add that manually, why worry. Just put your demo version out there, so you can actually have people back you early. Or how or why are they backing you in the first place...
In theory you could sell a text file as a place holder. But an early access version is better.
And of course there is several preorder mechanism and if this is your first paid project, do not try to switch your payment status days befor your release, or you might open another thread, why your game is not indexed.