Hello, welcome in the forum and thank you for your purchase!
Here is an additional answer 
No, there isn’t a big guide about migrating, the only HOWTO about Apple/iOS is this one about migrating Contacts to eCloud. You can find the complete list of HOWTOs here.
- Encrypt your phone (it’s already done by default on the FP3),
- Use a good password to lock your phone (only useful in case of a physical threat),
- Don’t root your phone (FYI the equivalent of rooting an Android device is jailbreaking an iOS device),
- Only install trusted applications, and not the first Calculator, Flash light or Fart app you find. Ideally, only install open-source apps that are on F-Droid (every F-Droid app is also available in “Apps”),
- Update your device to receive the latest security updates,
- Don’t let Wi-Fi or Bluetooth or NFC activated when it’s not needed.
If you respect this, you are safe (or at least you can’t do anything more).
For me, an antivirus on Android is useless. It’s just a proprietary app (not open source), with trackers built-in it, that will ask for a lot of permissions, spy on you and use RAM and battery. There is no risk (let’s say almost no risk because the NSA could send you one) of getting a virus if you respect what I said above.
Like other people said before me, you can use Aurora Store:
or request the app you want to use (banking apps might not be added even if requested) thanks to its codename (you will be able to see the codename of an app in Aurora Store, under its name).