This begs an importent question: how? How do you filter without putting up barriers that high quality contributors will not want to climb?

I ask both as a maintainer who has been overwhelmed and as someone who declines to participate in communities where the bar is set too high for casual drive-by contributions.

Make it a rule that any PR that changes over ... 100 LoC? 150? <some number you're comfortable with> needs to be broken down into smaller, more atomic ones.

If that can't be done, e.g. because it's a very large refactor that can only be published as a unit, get them to walk you through each design and architectural choice.

I don't think a high quality contributor will take issue with that.