Sharing a bit of my research and testing over the past several months as it relates to GPS denied position hold with PX4. Feel free to join the Discord here:

  / discord  

ZED Mini VIO:
https://store.stereolabs.com/products...
https://github.com/bellflight/AVR-202...

Bitcraze Flow Deck Optical Flow:
https://store.bitcraze.io/collections...
https://github.com/mavlink/MAVSDK-Pyt...

ARK Flow:
https://arkelectron.com/product/ark-f...
https://github.com/alireza787b/mavsdk...