I would be interested in brainstorming (opinions, discussions, experience, etc.) on low-cost monetization for essays and pamphlets regarding services like Patreon, Buy Me a Coffee, Leanpub, etc.

I can add ko-fi.com and gumroad.com to the list, if you didn't already count them under "etc". Everything I've heard from people who are on those sites indicates that it's very hard to make non-negligible money on them. Only a very small fraction of your audience will ever give you money. Incidentally, I've heard from those same sources that it's preferable to have a lot of low-paying patrons (or whatever) than a few big ones, so that your revenue stream is smoother. Either way you need a pretty big audience.

Thought it would be useful to share my view, thinking from the perspective of an eager consumer: I would totally pay for creative knowledge, be it in the form of essays, pamphlets, reports, diaries, notes, ideas, concepts, whatnot, especially material related to FoC. If builders and researchers in the community could package their science into small buyables, that could form a really nice market. I am thinking of cheap (say $3 ~ $7) but frequent. For instance, if a (V)PL creator has worked a lot on a particular design/feature/concept, if he could isolate it and wrap it into, say, a markdown with gifs, I would buy a few of these every month, and probably become a collector.