Oh - missed that one (did not expect more prizes on the questline) - that would mean about 3 trees more ~ 2 more upgrades - with "every 3rd prize is no upgrade" that would mean an average of 12 upgrades achievable (one more than needed)300 stars after completing task no 40...
That already sounds better even with not so good luck, full lvl 12 might be possible without diamonds (still depending on the prize line-up)