@Raydnt try:
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Watasi no Syounen — more of a melancholic surrogate parent relationship, also veering into age-gap romance.
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Musume no Tomodati — more like a surrogate child relationship that also veers into age-gap romance
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Kodomo no Zikan — significantly more lighthearted at times, but keeps similar dark themes about broken, dysfunctional families, with a lot of substitute-parent relationships.