The point is to make money from the users, not from advertising companies, which is the opposite of adblock. He could still refuse ads by ad tech, I don't see how donations by users would force him.
That's exactly the point. You want to be financially independent, or at least earn enough to live to reduce the risk of being corrupted by ad tech money.
"Freelance software development", problem solved. Not that the tax authority cares at all where the money comes from. They don't have a "cash income" category because they have a deep and burning desire to know these things. They're primarily busy with taking their cut.
Patreon aren't donations, they are recurring income in exchange of something (membership model with specific rewards per tier). In many jurisductions these must be declared differently.
If there is no money involved the project cannot be bribed by ad tech. The only acceptable ads are the ones the user white lists.