In theory, you are correct - you can combine spacers and springs to achieve the lift height you want.
However, the issue you are setting yourself up for is this - when you start getting into the 3.5"+ of lift, you're coming very near the end of your stock shock travel. My own observation - you get about 9 inches of travel in the stock shocks and you are pretty well center of the range stock. 3.5 inches of lift means you have about 1" of droop/down travel left on stock shocks - and that's really not great.
So, you swap to a longer travel shock and address the droop issue - the problem now is that, potentially, you're drooping your coils/spacers out of their seat because your shocks and out droop your springs. I suppose my point here is this - upgrade your shocks and springs together to make sure they continue to work together.
Using spacers to level out your Jeep, assuming you have a shock and spring setup that works together and didn't accomplish that for you, is a perfectly viable option.