I too have had troubles with multi-site P25 TRS radios. So far, I'm only using a 396XT with the latest firmware and default settings. I'm in NE Cobb county, and have a difficult time decoding ... Cobb county! Even with a full strength signal. Crazy.
Honestly, decoding this stuff is like the rules of Real Estate (location, location, location). If I travel a few miles from my home, or other parts of the city (even well outside of Cobb county), I get almost perfect decoding of their TRS. Our radios are getting confused with multiple binary 1's and 0's coming in at the same time, but from different sites. Possibly a little later in time, causing bad decoding. I guess the commercial radios have proprietary software that work out these problems.
Here at home where reception is not good, I have decoded my local TRS out on the back porch with NO antenna hooked up! The signals are that strong. But in my home, often the comms are very broken. Putting the radio horizontal often helps. Perhaps using a very small rubber (stub) duck antenna? I've also used an 800 MHz only antenna with good (but not very good) results. Anything to get your radio to only hear one site.
We can't comment on using a "better" antenna, because nobody knows. You have to try it out for yourself. Again here at home, I mounted a commercial grade 800 MHz omni directional antenna on the roof. Got it for free

Fed it with LMR-400, which is about 75 feet total. I'm certain it hears multiple sites up there, but for some reason, when I hook up my 396XT to that antenna on the roof, I get perfect decoding all the time. I'm not really sure why, but if I really want to listen in on my own county, I'll be down here in the radio room using the antenna on the roof! Perhaps something like that would work at your house too?
Good luck, and if you do find a combination of software settings that work better than the default, please let us all know.
Tim