In my opinion, these achievements should only require the specific chapter. I say this as someone who has bought each chapter since Elsweyr (when I started playing) and nearly always has ESO+, but to me it seems right and fitting for this sort of achievement, that someone who bought just the one chapter would be able to complete the main achievement. The items behind the leads should be their own rewards — that is even how it is introduced, go after that which interests you. Personally, I find it an additional frustration from unmitigated RNG: not only is the mythic dependent on chance, but so is the achievement and mount that I would like to unlock (still don’t have Savior of Necrom).