You are thinking well and asking good questions. Alas, even if the noun were feminine, it would still be «do que»; what would change is «o meu»: «A receita dela é melhor do que a minha.». «Da que» is never used. It is always «do que» because «o que» means "what," so the sentence above literally means, "The work/job of hers is better of what mine." So, "of what" translates to "than," and «o meu» is just the possessive form, that sometimes in Duolingo can be found without the article. Hope this helps.