rtl-sdr.com

Got a screenshot of what your spectrum looks like during a pass? You need to use NFM or WFM mode (WFM mode usually works better), and definitely not AM mode. The filter size should be just large enough to cover the signal, at least 34 kHz. So - the issues. When using RPi 4 with either of the dongles...

V3 Pro

This is a cloned unit, so I can't offer any support if it's a hardware issue. But even if it's working fine, you do need a properly sized HF antenna for it to work.

It shows two bulk in devices normally. Zero is the SDR/radio itself, One is the InfraRed endpoint which is not used for SDR. You don't need to do anything.

Are you maybe referring to KerberosSDR? Please ask in the appropriate forum if you are.

With dec and FIR filtering off, you should check the spectrum and see a single nice peak (assuming there are no other signals being received). If it's overloading from too much gain/signal, then you'll probably see multiple peaks with weird shapes. Also one more thing I forgot to mention, don't forg...

Are you trying to connect to the hotspot generated by the Pi, or have it connect to a hotspot generated by your phone?

Very nice! Have you managed to confirm DFing after adding those switches? No phase distortion?

First point is that the antennas you are using are way too small for 121 MHz. Coherent DFing can withstand quite a bit of tolerance in antenna element sizing, but you're currently using an 800 MHz antenna for 121 MHz, which is quite major. If you have them, use the long whips that came with the set ...

The software really depends what you want to use it for? The HackRF is mostly for hacking and experimenting, so GNU Radio.

Other than that, GQRX.

The loop will most likely work the best. A cheap Chinese loop is fine, but as you're in the UK look up Welbrook loops too. Obviously much higher cost, but really well built.