I spend a lot of my free time working on personal projects, typically sourced from kodeko, the android developer site, and other & mobile developer blogs.
I often find that these projects are good experience, but not in depth enough to deploy. Are there any resources(or advice for creating ideas & development plans) anyone would recommend for building intermediate and advanced applications?
Advanced applications are advanced because they package up a bunch of functionality in a clean, cohesive way. In some cases, the actual code you write has to be "advanced" in terms of clever programming or performance tricks, but that's usually not the case.
Given this, since you've already built some basic mobile apps using kodeko, my argument is that you already have the necessary ingredients to build advanced applications. Just start with an existing app and extend it! Add more design flair, make specific use cases easier, or combine 2 disparate ideas in one app (e.g. score keeper for pickleball).
The other option is to observe how others build Android apps, and either contribute or copy them.
Alex open sources almost all the mobile apps he has built (and he's built more than 30!). Links to the most recent repos:
I also have a few Android repos you can take a look at, like this Memory Game app: https://github.com/rpandey1234/MyMemory
Here's the masterclass on building meaningful side projects: https://www.jointaro.com/lesson/OEuX0KMhcnNSTJ50Voee/masterclass-how-to-build-and-grow-tech-products-to-500k-users-for-free/