First things first - and you're using DSD+ aka DSDPlus, not DSD, there's a relatively large difference between them in many respects - I was going to immediately suggest using DSD+ anyway but noticed that's what you're using, so... on to other things:
- alter the Filter bandwidth to 12.5 kHz, or even better perhaps 8 kHz - it shouldn't matter but definitely use one or the other and not the 16.5 kHz you're currently set for - with SDR-Radio (another SDR program) there's a known issue with using anything less than 12.5 kHz for P25 decodes, but with SDR# it's not an issue, at least for me, and I just use 8000 for every type of FM-based signal without issues
- set the Filter order to 10 (you can try other settings but that's what I recommend and use myself and have very little if any trouble with decodes, ever)
- check the Correct IQ box (this is pretty much mandatory, always)
- Step size is fine so no issue there
- Uncheck the Filter audio box - this one thing more so than just about anything else (aside from Correct IQ) will immediately improve things
- Audio volume looks ok based on the AF Gain you've chosen, might try setting it squarely on the second to last notch (there are tiny little marks there, just hard to see sometimes, that's basically the 80% marker)
Those should get your decodes working noticeably better off the bat with just a few checkboxes. Now here's a tip or two to make things easier to visually deal with:
- your noise floor appears to be roughly -60 dB like most every "cheap USB TV tuner" stick out there (mine gets me -65 to -70 dB but I've gone to great lengths in terms of RFI reduction with chokes and torroids and shielding) so I recommend/suggest that you adjust the FFT range so that it basically uses -60 dB as the "floor" itself meaning the entire scale of the spectrum is from -60 to 0 dB - you'll still see the peaks of noticeable signals and anything that's at or below that level basically gets drowned out by the noise anyway
- disable the RTL AGC and Tuner AGC if they are enabled (uncheck both options) - use just the RF Gain slider and see how things work with it around 28 dB. You can adjust it if needed but these RTL sticks based on the R820T tuner (RTL comes from the RTL2832U communications controller chip but the tuner is the Rafael R820T) are sensitive to front end overload meaning in areas of very strong signal saturation (say that fast 3x, I dare ya) you'll encounter a ton of intermod distortion as well as severe ghosting of signals that don't actually exist on the frequencies they appear to be on
The fact that your waterfall is set to about the middle of the scale (based on the slider on the right side for Contrast) and yet the waterfall itself is "overloaded" with yellow (noise basically) and red (actual signals) tells me you've either got the RTL AGC enabled, the Tuner AGC enabled, or perhaps both of them simultaneously and that's a very bad thing: you're overloading the tuner front end and it's visibly happening because of that condition displayed on the waterfall.
I recommend to people that they set the RF Gain with the slider and the Contrast of the waterfall so they see the signals they're looking for clearly and the background noise (represented by all that yellow in your situation) isn't actually there and basically you're trying to get a very dark background so the signals just kinda "pop out" visually.
It's entirely up to you how you want things visualized, of course, but in my experience of using this visualized form of seeing activity across a spectrum nowadays leaves me not caring about the noise at all and just wanting to see the signals, if that makes sense.
Try those tips above to get a better decode and then report back. Just unchecking the Filter audio box alone should have the most dramatic effect, but I'm pretty sure you've got the RTL AGC and Tuner AGC enabled and it's basically killing the tuner in the process.
Also, the AGC section on the left side is only active when you're tuning AM signals (including SSB obviously) so, you can "close" that section unless you must have it being displayed. It does get a bit cramped over there in that frame on the left side, so space is valuable in terms of cramming as much useful info there as possible.
