Azure SQL may not be the database you're using, as you think it is way too expensive, and really not flexible or scalable enough for your new project. Let me show you three small, incredibly precious, gems hidden in Azure SQL engine core that will at least make you think.
I'll show you how much less code you have to write to create an API solution that can be accessed at scale, that supports completely secured access to data stored in the database it allows users to access to, and that also provides support for intelligent data syncing so that the API will can provide a list of changed data since a user access it last time.
If that's not amazing I honestly don't know that it is. Oh yeah, everything will work with any platform and language, even though I'll show samples only using Python and .NET Core.