I'm going to be doing a similar writeup as my app launches in 2 weeks. I've got a small budget. All the design+dev was me, but I'm spending about $300-$400 on marketing. We'll see how that goes.

If anything I believe you should be spending more on marketing nowadays. There's no real way, at least to my knowledge, to search for a niche hole with keywords. I do, however, like to think of an app idea and then spend an hour search various keywords and seeing the results/competition. https://appstorerankings.net/ is a good website to find the keywords of an app. I combined common keywords in the app that I'm releasing with common keywords that are missing among competitors that yield few results.

Again, I haven't launched this app (my 7th) yet, but I'll be doing some analysis of its success using the marketing budget and keyword research.