Y'all ever pre-make a batch of old fashioned

Absolutely. I have done it a few times before a party. Saves a lot of time and less mess.

Would this be simple syrup and bitters premixed into the bourbon? Then poured over ice.

(Vs just bourbon and simple syrup premixed)

I prep gallons every year for Mardi Gras. Works out really well. I keep them refrigerated in half-gallon glass growlers. Pull them as needed.

Pour over ice, add your prepped citrus peels of choice and a Luxardo cherry, and you're in business.

Somebody give this guy the recipe/proportions for a large batch.