Ive got the exact same issue at my location.
Maybe install an amplifier ?
Crank the output up to the point you have 50 watts leaving the duplexer... and upgrade the coax too.
I've gotten improvement just going from 35ft of LMR400 to 30ft of LMR600 on my repeater setup. My repeater pushes 48 watts into the duplexer (A Bp/Br cavity style), which sends 34 watts to the Comet GP6NC (9db gain) GMRS antenna.