Should I build my app on iOS or Android?
You can build on all systems at the same time using Flutter, React Native etc.
Backend you can write as an API in some stable technology like Java Spring, but Python Flask should be enough.
Also, think about deploying that app as a website, so it can also target desktop users.
You can go independent from start and focus on marketing via videos on YouTube + top UX based SEO content.
Then if your app picks up, you can add it to Google Play.
I wouldn't do anything more for start.
If you worry about security, I found that a few people are involved in hacking these days.
It's because we've hacked monetary system and can print money at home without doing actual hacking.
Our monetary system is not working to be honest. It was hacked by Google and Meta.
It only works for those who are versatile and can deliver products without worries about failure.
So people who would normally be hackers if they lived in the 80's and 90's, they're hacking people's brain and algorithms now.
The most hacked thing on the internet is WordPress, so just don't use that in your operations for safety. Most black hat hackers hack things automatically.