I agree with your assessment of the actor's makeup decisions, but not your assessment of the writing.
Kira was not the only reason Odo did not rejoin the Founders. Sure, it was the clinching reason, but he also strongly disagreed with their militaristic dominance of solids. If it weren't for Kira, he would have gone to his homeworld, but he would have tried to spread the gospel of peace just as he finally did at the end of the series. And what makes you think Odo's interest in Kira is sexual rather than just intimately personal? Odo seemed to enjoy sexual relationships for the personal intimacy and closeness, rather than the physical pleasure.
Not only that, but it's not as if the Founders were always there, welcoming him with open arms. They were the ones who abandoned him (and the rest of the hundred) to the universe, they often treated him with pity for his attachment to the solids, they turned him into a human, they went to great lengths to deceive him multiple times, and the female Founder left him behind on DS9 when Sisko retook the station.
Not exactly the most welcoming family dynamic to go home to. Though Kira was Odo's primary motivation for staying on DS9, I always figured his reasons for not returning home were more complex.
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