Upgrade the MA, probably best, imo. Expansions are also good, but I will wait for the Black Firday sale when they are offered at a 20% discount. Win your magic residences and workshops in the spire, then upgrade them with blueprints (good for all but the chapter 3 variant). There may come a time when you run out of blueprints (if you won plenty and raced through the chapters), diamonds make up for that. One extra builder is more than enough, timeboosters can help a lot. 4 or 5 builders are just standing idle most of the time. I consider everything that is not at least semi permanent a waste of diamonds, so I will never use them to speed things up. The effects of the cauldron are just too insignificant to justify spending diamonds imo.