An instant reply can express availability without expressing attention.

Speed and attention are separate things, and speed is far easier to supply. A reply arriving in four seconds proves someone had a phone in their hand, and that is all it proves, since a real answer usually needs a moment of thought that four seconds cannot contain. Being permanently reachable can even become a way of avoiding the slower kind of presence. A fast response feels responsive while being almost empty.

Let one message wait this week until you can answer it properly. Say when you will come back to it, then come back to it.