How can you afford to do all these Level 5 managers that you mention, festival, ferry, and fireworks, that's a lot of pots of gold.
Gold is unlimited, you can buy what is required with a credit card. I know that many think this is a big no no, I on the other hand think that without people supporting the game there would be no game. Plus the cost is less than one night at a tavern and I'm at home protected from a DUI.
With that said I don't upgrade all the managers, the only ones that benefit finishing the city are the the ferry, festival manager and fireworks, I do drinks too but don't believe it is needed if you are willing to take another hour or so to finish.. The rest only upgrade if they are one of the tasks. I've done 6 cities so far and experimented with all of them just for fun. Unless you get level 5 managers for ferry and festival your max shamrocks production is severely restricted and it will take several days to finish the city. With level 5 F&F (Ferry and Festival) it is pretty easy to hit 7 Q/8hr and finish a city in less than 12 hours. Regardless of if you choose to use the managers at high level or not the strategy to get thru is still the same.
Key strategy is still to level ferry and festival to the multiplier point then raise the building production to the multiplier, then build the next production building. I cannot image doing the city without a manager for each production building, they automate the process so you can go to sleep.
Ferry and festival manager: rate multipliers at these levels:
x2 at level 10, x4 lvl 25, x8 lvl 50, x16 lvl 100, x32 lvl 150, x64 lvl 200, x4 level 250
Production building rate multiplier at these levels:
x2 at lvl 25, x4 lvl 50, x8 lvl 100
Basic strategy:
1> Start hats, level to 10, buy managers for ferry, festival, hats, alternate leveling ferry and festival to 10, 2x multiplier point and hats to lvl25 2x.
2>level ferry and festival (F&F) to multipler point, build next production building, level it to 1st multipler or until it fills ferry and festival manager. Repeat step 2 until fireworks is complete. Balance F&F rate by increasing fireworks until ferry is always full. You can watch , if your festival isn't keeping up increase it by a few levels (5 to 10) above ferry, don't balance it by the production rate shown in the circle, the times are different for the ferry and festival manager. I image you can follow this strategy and keep the managers at lower levels. several of the task require leveling up managers, I only level the hats, flowers, cake above L1 manager because it is one of the tasks.
At F&F level 200 overall rate is around 2Q/8hr, level 250 6.5Q/hours, the various levels between multiplier points don't make much difference in the overall Q rate. I built one city with the minimum managers as indicated the tasks, had festival 4, ferry 3 and drinks and fireworks 2, max Q rate with level 250 F&F was 2.09Q/8hr, 32 hours to finish, once the managers were upgraded to level 5 the rate was 6.72Q/8hrs, 9 hours to finish.
Increasing F&F to 300 ends up taking longer to complete as the cost outweighs the new time required
Keep in mind my whole objective was to figure out how to finish the city the fastest, this may or may not be best for you. On my last city I let it run until I finished all the tasks, it took an additional 8 hours to do this.