There have been plenty of directory services lately. During that same period, I also launched indieproducts.io. Luckily, it has grown organically, and I get at least two to three submissions every day (I approve one product a day).

Other than simply charging for submissions, what other ways could I monetize this service?

e.g. - Top spots(top 1 to 3 rows) for x number of weeks or months? - Write blogs and YouTube reviews for the paid submission? - What else could I offer as a paid service?

Who selects (filters) their suppliers by how much revenue they make? Seeing a startup makes no or very little money gives the impression they might not be around for long. At least in the B2B space.