the ppm correction has nearly zero evidence to ads-b - so forget about this item. your scan looks extremly ‘loud’ - what was your gain setting? as you have very strong signals at 960mhz (cellphone downstream) the external flightaware filter is worth nothing to solve your problems. you can try to move the ads-b antenna on the roof away from the transmitters and use a real good filter e.g. the 3-pole(cavity) i mentioned above. moreover i’d go for an unamped and unfiltered dongle.
=> 3-pole filter → lna → dongle
p.s. what dongle did you use for the scan?
p.p.s maybe this thread is helpful in your situation: The Final Filter Shootout