Just jumping in here to look at some possibilities. As long as your attic is not under a metal roof, you will get reception of ADSB signals, with some loss due to the materials of the roof. Rain will decrease the signals too. My primary antenna is in my attic at about 8 meters / 25 ft above the ground level. LOTS of trees all around me too.
Shortening the antenna cable reduced the signal loss, always a good thing. Your experiment with the receiver dongle at the base of the antenna could be a problem with the connectors or interference from the receiver, the PI or the power supply making noise at the ADSB antenna, just thinking and writing here. One thing that will help is installing Graph1090 on your system. Great diagnostic tool. Graphs for dump1090 -- my version with install script
One thing to be aware of, not all SMA connectors are alike. There are some with the center pin and others that have no pin. That could easily account for the direct connection losses. The no pin on the male connectors are know as RP-SMA, Reverse Polarity. Probably not the problem, but it has messed up people as we play with our systems to improve as you are doing.
Maybe this can give you some ideas to chase down. The FA ProStick Plus with the filter and LNA is a good product and might give you a good way to extend your range.
Have fun with improving your system. Best wishes always.