Best developing framework for small android app?

Hello everyone,
I'm new here and first I want to thank you for becoming a part of this community. I hope I will learn a lot here and maybe also contribute with my experience and knowledge to this community.

I am a software developer. I'm Java-based Developer and I have skills with Java (inkl. Spring/Hibernate), SQL, Angular 8, HTML/CSS and I have some small begginers experience with Android Studio+Kotlin (I'm able to make some really simple app which is not gonna have perfect design and maybe have some bug and code behind the app will be a spaghetti :D )

I have an app idea and want to develop it. Regarding it, I would have few questions. I would be really thankful, if I could get some answers.

The app should be simple.

• It expects some user's input and saves in database
• It does calculations and statistics of saved data and represents it to user
• It does maybe need some notifications
• It needs nice design and usability
• It should enough to have local embedded database, but is isn't a must
• For beggining, It does not neccesarry need any internet connnection like: backend server, authentication, database on the internet
• It should be free of maintanance cost for me

I'm searching for the best programming languages and framework to develop it. I'm thinking about Anroid Studio, because I already have some experience there and want to develop the app first for android. But, I also think about Flutter, ReactNative, Iconic.
Do you have some suggestions?

Also the database is a question for me, originally I wanted to make an embedded SQLite database, but now I started to thinking about the Firebase and its features for database and authentication.
Do you have some experience with Firebase and its pricing?

I would like to say a big "thank you" for reading my long post and thank you in advance if you have some aswers for me.
Have a nice day.