I think the discussions feature might be a little more comfortable for more people to express gratitude, but like you said even that isn’t explicit enough. I also feel disinclined to post gratitude for fear it’ll register as noise.

IMO since GH tends to be somewhat aware of how people piggyback their platform, a README gratitude badge might be a good place to start. It can be as simple as a button click plus an optional prompt to add commentary. In fact, I might make this a smol project to see if it gets some traction.