Honestly, it sounds like you are too far away to pick up multiple counties unless your discone is pretty high. Second, P25 and other digital modes are packet like info. The radio will not properly decode partial transmissions. If the transmission were analog, you would likely be able to hear them but they would likely be noisy. You don't get noise with digital modes, you get dropouts of packet segments making it hard to decode if not impossible. It is also likely that the VHF Low frequencies (30-160) are traveling better and farther because at lower frequencies (compared to 700-800 MHz) the radio waves are less absorbed by surfaces like trees, less reflected by buildings, etc. At 700-800 MHz you are approaching microwaves and they bounce, scatter and are absorbed by all sorts of things including moisture in the atmosphere.
If you really want to pick up transmissions from multiple counties you are likely looking for an antenna with a gain factor. A discone has NO gain and is actually has a slight loss. That being said, many of us use them for all sorts of things at great distances, especially aircraft monitoring. A beam antenna has gain but will need aimed at the specific area you are looking for at any given time. A vertical with gain can be found for public safety but they tend to run in bands. Finding a single vertical antenna with gain for VHF Low, VHF High, UHF and 700-800 MHz is probably not going to happen easily or readily.
As some have suggested, a mast mounted pre-amp is the best bet here. If you can't do that, a pre-amp in your radio room may still help out. The idea however is amplify the signal close to the antenna, not at the end of your coax where it is already a bit weak. For the sake of going inexpensive, you can try a television signal amplifier. They generally have the frequency range to cover about everything you seem to be looking for. Get a decent one though and only as many output ports as you need and don't go with some extreme amplification factor.
Lastly, better cables allow more signal to reach the radio. Makes sense. You can read up on attenuation of signals when using different cables and it usually gives a dB loss for a 100' run. Less (loss) equals more (signal) in this case.
See if any of that helps.
S.