I haven't asked for an extended holiday. I'm worried about making such a big ask of my boss (our culture is not very progressive) and then coming back only to quit. What do you think?
Why is an extended holiday such a big ask? - would they rather give you that or lose you to ill health? What you could do is just quit, take a few months off then start applying for a new job. I would have thought someone with your level of experience could easily find a new gig. Only you know your exact situation.