When working in a busy shop there is NO TIME to let a cake rest overnight to 24 hours
Making a stiff dam is one of the best things you can do. Do not fill more than 1/4" thick.
Old school of thumb was to use a bag with only a coupler (no tip) but that give too much of a dam and most people will fill right up to the top of a dam. Use a tip 12 OR roll your thickened icing only as thick as what one would pipe w/a #12 - no thicker. Also perhaps you need a thicker filling. You don't say what you used but if it was something like a thin jam it will spread w/the weight of the layer on top of it. Another way is to torte each 2" layer into 3 pieces and use just a thin smear of filling on each. To do that you must have a very good leveler
This will give each bite of cake lots of taste and moisture from filling w/o ever having buldge problems.
One last consideration - how thick is your layer of fondant? You might need to roll it thinner so it is not so heavy - won't pull/weight the cake down.