Revenue model suggestion

Hi,

We are developing a flutter based app for IOS and Android for a Niche Category. It is a calculator and convertor which is specific to the Jewellery Industry. I wanted to understand what is the preferred revenue generation model one should go with

• Freemium where free version contains ads
• Subscriptions based model ( Monthly or Yearly)
• Paid App i.e. direct one time purchase

Also, considering the Fortnite app store saga is it advisable to collect subscription payment outside the App store and Play store system ?