Since a randomized, double-blind trial is impossible (or at least financially impractical and incompatible with the self-help coach's business model), what do you consider a reasonable standard of evidence?

A reasonable standard of evidence is established by what it takes to change your mind (ideally you'd need to work from elicited prior, which allows to check how reasonable your requirements are). If it's double-blind trial that is required to change your mind, too bad it's unavailable.