The whole narration around “contributing”, “PR welcome!” in OSS completely eluded the fact that making a PR is only a tiny and the easiest fraction of the work needed to integrate suggestions from random people from the internet into a cohesive result.
Maybe one thing you could try to add is a kind of tit for tat statement in CONTRIBUTING.md. Something of the form (better said) we are happy to take you work, but integrating and maintaining it takes time and ressources, there is a social expectation that for any PR merged of yours you will make a full review of another PR.