I recently created this music visualizer app called Superplay as a side project. It helps artists promote their music and create content for socials media. I was burnt out from a larger app I was working on, and wanted to build something simple over a couple weeks. I realized that video rendering could be done using the phone's screen recording function instead of having to render out video on a server. This was a huge breakthrough as I was then able to not have a back end, and therefor no costs to run the product. Apple obviously takes a small cut, but that's a given. Having a product that is pure profit and requires almost no maintenance is a game-changer. There are also no accounts as apple handles payments and I store user's files locally. It's a massive weight off my shoulders and I'd recommend this approach to anyone if you can do it in your niche.
I've found a few channels that have started to work for marketing and am looking forward to growing the product more. Here's a link if you want to check it out. https://www.getsuperplay.com