Thank you so much for the very detailed response. I'm sorry for taking so long but I finally got some time to try messing around with it again.
Edit: You mentioned trying two different controllers. Have you tried to have both plugged in at the same time so that the game can see both?
So I actually use the controllers via Bluetooth because I couldn't get my Mac to recognize the controllers when plugged in. I'll try a few more things and see if I can get it to even get recognized.
While you have the game running, are you able to open the Steam Overlay?
I wasn't able to get Steam Overlay to show up while playing Stardew Valley. Tried with controllers and with mouse/keyboard and it didn't work. I confirmed that it worked with other games.