I was always a fan of not per stream but of percentage based minutes listened. I spend 15€ every month, take off taxes and Spotify operating expenses, which would be like 10€ left: - I listen to X 90% of my total listening minutes this month, so they get 9€.

- I listen to Y 10% of my total listening minutes this month, so they get 1€

I think this would be fair, because I kinda listen the same minutes every month, and most people with a fixed daily schedule probably do it too.