illness
恙 centers on illness, specifically a kind of sickness or ailment. The common phrase つつがなく (without illness) extends this to the idea of being free from trouble or harm.
恙 combines 心 (heart/mind) with a phonetic element above that is related to 羊 and 王. The character originally referred to a specific illness, possibly one affecting the mind or spirit, but the exact historical development is uncertain.
The bottom is 心 (heart), and the top looks like a tangled mess. Imagine an illness that tangles up your heart and mind, making you feel sick.
For ヨウ, picture a sick person slowly drinking a cup of yogurt to soothe their illness: yogurt -> ヨウ, and the yogurt helps calm the tangled sickness inside.