boom, prosper, copulate
盛 centers on fullness and vigorous growth: a boom, prosperity, or something at its peak. The copulate sense is a specific biological application of that same idea of flourishing and reproductive vigor.
盛 combines 成 (to become, accomplish) with 皿 (dish, vessel). The idea is likely of a vessel filled to the brim, expressing abundance and prosperity. The phonetic contribution of 成 is uncertain.
Picture a dish (皿) that has become (成) so full that it overflows with abundance. That overflowing fullness is prosperity and booming growth.
For セイ, imagine a booming business that says "Say yes!" to every opportunity: "Say" -> セイ, and the prosperity grows.
height (e.g. of summer); peak (e.g. of cherry blossom season); (in) season; full bloom; full swing (of a party, etc.)
climax; uprush
prosperous; flourishing; thriving; successful
to serve (in a bowl, on a plate, etc.); to dish out; to dish up; to fill (a bowl) with
to swell; to rise; to bulge; to be piled up
grand; magnificent; lavish; large scale; prosperous; thriving; lively
lively; vigorous; energetic; healthy; avid (e.g. desire); rich (e.g. imagination)
to pile up; to heap up
large serving; large portion
success; prosperity; boom
prosperity; flourishing; thriving
prosperity; flourishing; thriving
height of prosperity
to incorporate; to include
arrangement of food (on a dish); serving of (arranged) food
graduations (on a ruler, thermometer, etc.); gradations; division; scale
rise and fall; ups and downs; welfare; vicissitudes
flowers in full bloom; time of year in which flowers are in full bloom
prime of one's working life
in high spirits
golden age; prime; heyday; height of prosperity