It appears to me you should focus initially on upgrading the festival and the ferry managers and building levels, to finish in a reasonable time you need level 250 level ferry and festival buildings and level 3 managers for these two, work them up gradually, the cost for the building upgrades is much less than the production buildings, pay attention to the next task and follow the tasks closely, don't over spend trying to upgrade a production build to the task level until you can easily cover it with the festival and ferry. Try to save upgrades to buildings when you have a task of upgrade buildings xxx times. The higher production buildings are most important because they produce exponentially more shamrocks, try to keep the 1st three at relatively low levels, only upgrade them if you have a task to upgrade them, then only do so if you need shams production increase. The trick is to balance your production rate with your ferry rate and festival output for best efficiency. You can't make more shams than the festival guy can sell nor the ferry can haul, so there is not much point in having large production unless you can haul and sell it. TO finish a city in about 8 hours takes festival and ferry managers of level 5 and their buildings a little above 250. To finish in less than 8 hours raise ferry and festival buildings to 300.
festival, ferry mgrs Level 5,
The only other manager at level 5 is the fireworks manager, the first 4 production buildings don't really add much to your 8.4Q shamrocks, you only need them to get started. Each building produces exponentially more shamrocks so Fireworks is the one you need to get build.
Note: use the Time Skip button to check your production rate and get an estimate of when your city will finish. Last comment: once you have 5 production buildings and get a task that says Upgrade Flowers/ Cake/any of them to XXX level be very careful, While it looks like you have plenty of shamrocks to make to complete the upgrade I have used ALL my shams trying to complete this task. It is very deceiving, The amount goes up in a linear fashion but increasing a number 75 times at 10% each time ends up being a very large number. It is fine to do this earlier in the city but towards the end it will lengthen the completion time. Best to not complete this task and go for the end.