Soft dinner rolls always taste bland or flat
You've answered this question yourself.

Flavor in bread comes from:


salt
yeast action


and to a lesser degree 


fat  
sugar


Obviously you have plenty of fat and sugar.
As you suspect, your yeasty flavor will be improved by letting it rise longer in the fridge.

Flat flavor in bread is almost always because of not enough salt. I agree with you that you don't have enough.
As the saying goes: "Bread without salt is insipid"

For how much salt is good, see this question: Amount of salt in bread  

About 2% of the flour weight. In your recipe with 5 cups of flour (600g) that would be just over 2tsp of table salt (12g). 

I use kosher salt and would probably use closer to a tablespoon for a batch of bread this size.