Thanks! I don't think you can easily set up a "give a key to people who bought it elsewhere" system on Steam so holding off for now is probably your best bet.
If you’d like to, you can set it up on itch.io so that everyone that pays for the game gets access to a Steam key. From what I understand, this also allows anyone who has previously paid for the game to claim a Steam key.
You can find more details here: Download keys - External keys