I think your strategy to gather coins is perfect. Even though you have several characters collecting, there are probably several you don't have that are work horses for coins.....like Elsa has an 8 hour task, Daisy 4 hours, Jack/Sally 8 hours, Sully 4 hours, etc. In addition, the more leveled up a player is, the more options they may have for tasks to gather tokens. Jessie and Woody both have short tasks AND long tasks to gather coins. So whereas you might only be able to send them to get 12 coins for one hour overnight, others can send them fir 8 hours to get much more.
Good luck getting as many as you can. You're already a long way toward getting several gems at the conversion if they do the conversion this time.