I have about 10 quests left(not counting daily missions), 9 buildings and I had gotten some extra currency from 2% ranking in the wishing stall event.

However, I have been stuck on 12 cookies for a while due to dupes.

If dupes were not a factor then the chance to collect all the cookies is much higher but since dupes exist a person has to contend with every guaranteed cookie pull crossing 10/20 having less than 50% to be a new one. The percentage decreases as 10 goes to 11 to 12 and so on.

When you reach the 16th cookie or so it is much harder not get dupes so the remaining countermeasure is saving wishing stars from dupes. It takes 8 cookie dupes to redeem a new character(unless a few decor dupes are also gained).

The probability of getting 15 unique cookies from gacha alone is 3.0518 x 10^(-20) to 4 d.p. or 0.00000000000000030518. It definitely varies from person to person but you still need to be pretty lucky to get close to 20 cookies.