I agree with your main point, but to clarify: Flattr was specifically designed for this sort of one-off usage. Clicking a Flattr button doesn't subscribe to anything, it just adds that person/site to a list on your Flattr account. At the end of the month, your money is divided evenly between those on the list, then the list is cleared.
You can set up subscriptions on Flattr, which sounds more like Patreon's model, but the default mode is for one-off drive-by donations. Flattr subscriptions are just an automated way to click a Flattr button once a month.