Analysing performance of Sub-1 GHz 433 Band
For the actual Radio Frequency measurements, I would strongly recommend using the device itself to measure signal strength. It already has all the features you need, it's available, and it's representative of the system you will deploy in the end.

Have a special mode in the firmware that sends and receives and reports the signal strength back to you. Using a spectrum analyser will be more difficult, and much more than $300.

You can also use the test system to investigate BER vs signal strength etc.

Once you have a measurement of the RF paths you can work on the networking questions, in simulation not in hardware. This is far outside my area...