You do not need a full SDR to make a waterfall. The nrf52840 can function as a coarse spectrum analyzer, because it lets you tune to any frequency within its range, and then measure the approximate received signal strength over a period of time[0]. I have tested this myself, and it works pretty well. The only downside compared to an SDR is that it can only listen to one frequency at a time, so it will sometimes miss short signals.
I know it can switch frequencies, but I predict it wouldn’t make a very pleasing visualization if the updates are slow, but maybe you are right it would look fine for this implementation which is already far off from a waterfall
