imperial throne, happiness
祚 centers on a blessing or fortune granted by heaven, specifically the divine right to rule. The imperial throne sense is the highest form of such happiness, while the broader happiness sense extends to general well-being bestowed from above.
祚 combines 示 (altar, divine) with 乍, which likely contributes the sound and may relate to the idea of making or creating, suggesting a divinely created blessing or position.
The left side 示 represents an altar or divine presence. The right side 乍 looks like a person kneeling to receive something. Picture a ruler kneeling at an altar to receive heaven's blessing and the imperial throne.
For ソ, imagine the sound of a soft, solemn sigh as the emperor kneels at the altar to receive the throne: so -> ソ.