We sent in Boarding passes, any receipts from hotels restaurants, photos of us together throughout the trip (try not to wear the same clothes all of the time). We sent in a screenshot of 1 WhatsApp message for every month (preferably with us swapping a photo, timestamped). A picture or 2 with some of her family members, yes, you should both actually be in the photos. Western union receipts as I would send myself money as needed and on occasion send her money for some expenses because she was not able to work her job during Covid lockdowns.
In all honesty what we sent was fairly minimal compared to what I have seen some on YouTube videos who showed their packets. We had ZERO issues and no RFE's