What I've seen some small websites do is practically sell / rent backlinks. I can imagine this having the downside of doing business with people on the internet

What I've seen some small websites do is practically sell / rent backlinks. I can imagine this having the downside of doing business with people on the internet, not getting paid. A slightly more ethical / more profitable / safer option would be to make websites in related niches (tech reviews, for example) which are easier to monetize, and then link to those. The costs of running the free websites would be paid by the profitable website. Apple does a somewhat similar thing. iOS and macOS are free, but they are covered by the cost of the device. Or an open source library (free) paid by a book (charging for the documentation).