Why Japan could not add another major operation before Midway

What additional targets could Japan realistically attack before Midway?

Very few, because the Midway operation required fleet concentration and preparation; extra operations would delay or dilute the main effort.

Japan would have little room for additional operations once it committed to the Midway plan. Forces dispersed on other missions first had to be brought together. Returning ships needed repairs and refits before they could join the concentration. More attacks elsewhere would therefore delay the assembly required for Midway.