CartographyAssets or the Roll20 marketplace or Patreon or small Kickstarters for things you were already going to make are probably the way to go. I recommend not assuming you can just know what RPG people need without getting into the hobby though, while yes it is how to get passive income your things won't sell well if you don't actually understand what RPG DMs need for their games. Don't just casually play a few games. Run a game and then make things based on what you thought and felt you needed. It's the best way to actually make a viable passive income product. Most of my stuff I made because I wanted it first and figured it would sell second.

If you look at my signature you'll see my links, and I will say, I make $300-$600 a month passively now between Roll20 and Cartography Assets alone. Well, this month might even hit $700 with the Roll20 marketplace anniversary sale packs, that's really boosted my income there, about 30% higher than typical. You don't need a ton of things, just things that will periodically sell. But, you might do better selling those pngs and brush sets, since you're focused on novel-style mapping, and sure, those kind of things sell too. Artstation marketplace is one place you can sell them, Designcuts is another.