I've added two new feeders to your network today, I don't know how useful they'll be as I suspect this area is already well covered. I have two receivers at home here, one is an Airspy mini with an RTL-SDR LNA and an FA aerial mounted 10m AGL with a pretty decent view and the other is in my loft using an FA Prostickplus with a homebrew two element j-pole collinear.
Both my Pis are standard Raspbian Stretch Lite with dump1090-fa and your Installation was very straightforward and completed within a couple of minutes. Thank you for making it easy.
I already feed to FA, FR24, PlaneFinder and ADSBHub so am happy to add OpenSky.
To give an idea of the area I cover, here is my PlaneFinder range map.
I also run a Virtual Radar Server (link in signature).
ADS-B messages are "wrapped" in Mode S messages. Seems your AirSpy ignores all Mode S messages other than downlink format 17 (extended squitter), which is the downlink format used to transmit ADS-B messages. It's probably not the AirSpy dongle, but rather the SDR software you are using. Are you using a different SDR for your FlightStickPro?