After countless hours day and night, and two different experts gave up, I finally made it myself. I have to say, I was pretty desperate.
Solution? I went to interfaces on local pfsense, added some cryptic ovpnc to interfaces and added manually NAT-routes for all interfaces wlan, lan, opt1, opt2 etc (all allowed, every direction). For some reason, I don't know why, everything worked! I can ping in every direction as long as I'm on a LAN. Now I have to reduce the access again so that I don't have more open routes that needed.
Thanks for no help on this…