ear of grain, ripe grain
穟 centers on the mature fruiting head of a grain plant: an ear of grain, especially when ripe and ready for harvest.
穟 combines 禾 (grain plant) with 遂, which likely contributes the sound and may carry a sense of completion or following through, fitting the idea of grain reaching full ripeness.
The grain plant 禾 on the left is followed by 遂, suggesting the grain has completed its growth. Picture a stalk bending under the weight of a fully ripe ear of grain.
For スイ, imagine a sweet ear of ripe grain swaying in the breeze: sweet -> スイ, and the grain is ready to harvest.