For ὁ μακαρισμός, beyond the difficulty with the main stem " μακαρ" (discussed above), there really is another puzzle with the "ισ."
This is the same difference that changes "wise man" to "sophist," so it is a significant addition.
It is due to this addition of "ισ" that LSJ adds "pronouncing" to the definition of ὁ μακαρισμός, i.e., "pronouncing happy" which makes sense. It almost seems to mean "the hailing toward blessing" or "the pronouncement of blessedness."