Also, do you think the only way to get discovered (for indie devs) is to pick app ideas that will match nicely to keywords that users search for? If you don't have a marketing budget, is there any other way to get discovered?
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.
What's your app name?