I do not quite understand, why you "can't sell them directly". You can make a project and sell a wallpaper, a patch, a soundfile and whatever data files you like. You can add files to that project later for future games or make several such dlc projects.
If you want to advertise your Patreon and give incentive to join, ok, but you yourself suggested to only subscribe for a month to just get the files. So it did not sound like you are doing it for those reasons.
So the idea was, to have the price be twice or more that of your monthly subscription to still give incentive to join Patreon instead, but also give an alternative to people not wanting to register at Patreon (which would be explained in the description of the project). Also, when files are lost, one would have to subscribe again to Patreon to download them. Or when you would update the files for a work in progress.
You can make the project hidden and only link to it from the game description, if you like. DLC projects are not common, but totally valid on Itch. Some try things like those individually priced files, where they sell the game for 10 but make an additional file available to those that paid 15. But this has all sorts of disadvantages. Mostly that you can not add 5 to a previous payment. It is much cleaner to have the main game for 10 and the dlc for 5. You can bundle such projects together for easy buying (and saving on fees) or for sales. It is just not very practical to have projects for 1 $ because of fees.
Itch's pricing and project options are strange sometimes. And what is not often talked about, a buyer does not even need an Itch account to buy something. Developers only get feedback made by people that do have an account. There might be reasons why someone would not want to register at a site to buy adult games ;-)