You see that tree over there that you're barking up? It's the wrong one. There's nothing up there. Seriously.
You're contradicting yourself in your question.
You know there's no direct relationship between dBFS and dBSPL, but then you go on to hypothesize that there is.
You were right the first time.
The easiest way to look at this is that you are trying to compare two things with completely different and unrelated references. One being "Full Scale" and the other being "Sound Pressure Level". What's missing from your equation is a way of relating the two references. It's a big hole.
The only possible relationship would be that if you were to be able to measure a dBSPL value at a particular point in space due to a sound source being in the same room.... if that sound source were to reduce in level by 6 dB, you could reasonably expect the dBSPL level to reduce by a similar amount.
But, knowing nothing about the sound source, the room, the measurement device, the speaker and the position in the room, you're on a hiding to nothing I'm afraid.