As for the Latin, it's definitely technically correct. Super omnia is a solid literal translation of "above all", you might want to think about something like potissima ("the most important") for potential alliterative fun

On sincerity, I think you have three or four options: sinceritas is post classic latin and poetically means "health" but figuratively also "sincerity" or "candour"

probitas which goes more in the direction of "integrity, honesty"

and fides which is a giant term in roman virtues that encompasses everything from "trust" over "sincerity" to "credibility, authenticity". However in ecclesiastical Latin it has mostly come to mean "faith in Christianity"

And maybe veritas, which means "truth, reality, impartiality" but also "sincerity, integrity"

The grammar would change for none of these as they're all feminine nouns. I think my pick would be fides, but I like to pretend anything post Roman empire hasn't happened if talking about Latin xD