On that point, this post from @obj explains where the rule of thumb for upper signal strength comes from – the “keep -3 dBFS or stronger messages to around or below 5%”.
I’m sure I read somewhere on here that messages with a strength of around -30 dBFS are merging into background noise (at least for the Pro Stick Plus), so that could be considered a lower bound when analysing results from gain changes. But I can’t find a reference for it now, and I’ve seen some people post results with numbers weaker than that and doing okay.
Assuming good open antenna with visibility all around, would it be correct take -30 dBFS as a rule of thumb lower bound for RSSI? As you say, the ‘loudness’ only matters in analogue applications, and in this application it only matters in the sense that too loud overloads the SDR front end. If the digital message can be heard weakly then it’s as good as being heard strongly from dump1090’s point of view.