adzuki beans, thick
荅 centers on the adzuki bean plant and its pods, which are thick or dense with beans. The 'thick' sense extends naturally from the compact, clustered nature of the bean pods.
荅 combines the grass radical 艹 with 合, which likely contributes the sound. The character originally referred to a type of bean plant, with the 'thick' meaning possibly derived from the dense clustering of the beans.
The grass radical 艹 sits above 合, like a plant whose pods come together thickly. Picture adzuki bean pods packed tightly under the leaves.
For トウ, imagine a thick toe of adzuki bean paste: toe -> トウ, and the paste is dense and thick.