Maybe rebranding GitHub Sponsors as not just a means of giving money but one-off "thank you" messages would be a decent approach to encouraging gratitude in open source.
There's Ko-fi [1] which seems to be positioning itself pretty much as a low-pressure "buy someone a coffee" option for showing support more than as about the money. I'm very interested in to what extent that influences donations.
[1] https://ko-fi.com/