I see no problem in encouraging contributions and donations in your CONTRIBUTING.md (that shows when creating an issue). Especially in a large project.

But requiring a donation or code would mean that lots of bugs would go un-reported. Issues are just as much of a contribution to a project as code is.

Founders getting overwhelmed is solved by adding maintainers or transferring maintainership. Popular projects will be forked if the maintainer burns out and leaves.