All Rule Actions to upsert multi-select options instead of replacing them

I ahve 5 possible multi-select options.  I created a rule with 5 actions to upsert the row with each of the otpions when applicable.  When the Rule runs, it is replacing the multi-select with a single option instead of adding each values to it.  This means, to have all combinations of possibilities, I would need to write 31 case statements and have 31 actions for each of the possible sets of multi-select options.  This only gets exponentially larger if more options are available in the multi-select picklist options.  

• Single Selects (5): Each option on its own.

• Pairs (10): Combinations of any 2

• Trios (10): Combinations of any 3.

• Quartets (5): Combinations of any 4.

• The Full House (1): All 5 selected at once.