For one of my ADS-B trackers, I use a discone on the roof – it’s wideband, 54 - 1200 MHz roughly.
It’s fed by 9913 and has a Comet CF-413B duplexer on it in my office/ham shack.
The low-pass side of the CF-413B is 1 - 460 MHz more or less. That side goes to my Icom 2m/440 radio, occasionally switched to my HF-6m radio.
The high pass side is 840 - 1300 MHz, and goes to the ADS-B system through a FlightAware bandpass filter.
I ran a lot of tests, and didn’t see any dips in ADS-B performance while transmitting on either 2m or 440, at full power (around 40W). Haven’t done any high power transmitting through the discone on HF-6.
The Comet docs show attenuation probably exceeding 60dB below 50 MHz, and 50dB or better in the 2m and 440 ham bands. Follow that up with the FlightAware filter and you should be good at the 50 watt level on 2 and 440.
I also use high quality cables in the shack – no cheepie 8x or RG-58, and have good RF grounds.
Very satisfied with performance of the CF-413B for combined ADS-B and VHF/UHF work. You could try using it as a high pass filter, properly terminating the low-pass port, and running the high-pass port through the FLightAware filter to your ADSB system.