Cool, the you can try to navigate on Guided using Dronekit Python with the RPI.
Get ready to switch to loiter in case the quad get lost, but practice make good and you probably can make a mission similar to the Balloon Finder QuadCopter Object Tracking on a budget using just the Px4FLOW.
Keep us updated on theses experiments, I flew a couple of time opticalflow mode outdoor on loiter and guided, and it could ‘‘go somewhere and come back home’’ with an acceptable error margin. Beware of the TFMINI range == its just 6 meter outdoor and if you go beyound that it might get the quad jump in the air… in this case you need to switch back to stabilize …!!with throttle!!.. so it dont fall like a rock… once again, practice the manual takeover… I generally practice it before I switch to guided 
As for the SLAM, I am still working to make it stable enough so I can publish a new blog and hopefully having you to experiment with it as well.