Hi, I'll help if I can. Like you say, a good way to do it would be with paid ads. I searched for Admob, and it seems like the easiest way in Godot without major headaches. Your game would probably have to get a lot of downloads to make this pay out much, as Google ads, for example, don't pay that well AFAIK. Have you downloaded version 3.2 of Godot, as it claims to make the integration a bit easier without custom templates?
Is it easy to get your Godot stuff working on mobile and get it in the app store? I'm thinking about doing something similar myself. Feel free to PM me if you don't want to get off topic here.
Here's some paid releases on steam made with Godot that go from about .99 cents to 10 bucks or more. Make something great looking like a visual novel or some jazz and someone may buy it. There's a fee to get on Steam, but I think it's only $100. You'd still need to promote the game yourself somehow...
https://steamcommunity.com/app/404790/discussions/0/412448792354265655/