I would do a Mobile app. It has a lot of benefits:

- You control its release and marketing, so it's your show A to Z.

- You talk to users about new features and bugs, which is great experience learning how to deal with user requests.

- You can hook it up to a web API, or a free-tier AWS account, or Apple iCloud (if targeting iOS), which gives good experience for connecting to a web db.

- You can also store data locally in order to learn SQL syntax using SQLite, if you don't already know SQL.

- You create the icons, splash screen, screen captures for the appstore, etc, so it provides great UI design experience.

- It has a great developer community (at least iOS does, since I have experience there); people are very active and cool and fun.

You can develop a Mobile app on the side while keeping your current job. I did this and was able to pivot to a more interesting job space doing mobile. I learned a ton.