Welcome! Wappler is amazing. I’m not an expert, but I wanted to say hi and share my thoughts and get the responses flowing.
For #1, I recommend a paid API verification service for OTP, as you can’t allow the OTP to ever have any downtime or essentially your app is also down. Users may be in India but sign up with a number from a carrier or region you didn’t plan for, and it can be a real headache to fix – I decided it was worth the cost for OTP as a service. Plus, until you have experience to manage a 99.999% level of reliability for your OTP, a service provider using APIs to send/verify the OTP codes is what I’d suggest.
For #2, you don’t probably want to store your DB in Docker unless you’re storing it in an attached volume or unless the data is temporary and OK to lose at any moment. Otherwise, every time you or the host restart your Docker you’ll have a blank database. I don’t know what offerings serve India, but it is more difficult to find Postgresql than most other database types in general … so expect only a handful of choices when you look. I still use PSQL for many reasons, but be aware you probably won’t have many options of managed hosting for postgresql specifically. Unless you understand the ins-and-outs of database backups, restorations, and admin setup, I’d use a managed database service if at all possible until you have a solid setup.
For #3, you’ll want your hosting to be physically near your end users – it can help a lot to have your hosting in the same data center as the database you use, which can help you limit your providers if you go by that ethos when choosing.
Another thing to consider: You might look into the use cases more. Will users be mostly connected via cellular networking? Does the app need to work in offline situations, even for a few moments? If so, CouchDB and PouchDB might be worth a consideration. Postgresql is addictingly powerful, but unless I’m missing something it might not be strictly necessary for this kind of app.