I built a web app for a side project but its hard to SEO optimize it because there's not many words. It's mostly a utility tool project e.g. fitness tracker. Only blogs seem to rank really high on websites
I agree that content marketing (blogs) is the primary way to get SEO clicks, but this is a slow, painful process.
You could try to reduce the time it takes to write valuable articles using tools like ChatGPT, or something purpose-built like copy.ai.
I have only had success (and quite a lot of it) from ASO (app store optimization) for my Android app side projects. For that, it's much more straightforward as you get the title, description, etc (i.e. all the data is structured for you and you just need to cleanly plug in).
SEO is frankly hell. Everyone is trying to game the system and there's far more competitors. I wouldn't spend too much time trying to optimize for SEO. I talk more about this and other ways to get side project traffic here: "[Side Project] How to get more users + interviews?"
The most sustainable growth channel is word of mouth. Be user-obsessed with your first 100 users and hopefully you can get something going from there.