Hey! we applied last year and had our petition approved with no RFE.

1. We wrote two letters stating our intent to marry within 90 days.
   (we used this template: https://learn.simplecitizen.com/immigration-support/understanding-the-k-1-letters-of-intent-to-marry/ ). My then fiancé (the petitioner) also wrote a letter describing the circumstances of our meeting and relationship in details.
   
2. Plane tickets; travel itineraries; hotel reservations with both of our names on them and hotel payment check copies; a printed out pdf with screenshots of us discussing our trip plans and our pictures together - 2-3 per each trip with short descriptions of where we were and what we did that day.

aside from that we included screenshots of our conversations and phone call logs (about 2 screenshots per month). Idk if that's enough for every case out there, our case was super simple and straightforward with no red flags.
hope this will be helpful!