I offer here a different alternative, which is very motto-prone, but a rather free translation of your original sentence:
Quis ut Deus? Who is like God?
It is the meaning of the name Michael (Hebrew מִיכָאֵל). The phrase in its popular Latin form is not literally attested in the Bible, but is present in the Church Fathers (e.g. St. Gregory the Great, Hom. 34.9, c. 593 AD) and is a paraphrase of several Biblical passages. You can find it as an inscription in a huge variety of places, from Canada to Germany to France to Portugal.
Although it is not a literal translation of your original sentence, it is quite popular and has been used before as a motto (more examples, and yet one more example of someone not called Michael). As this site puts it, "this is a rhetorical question, implying no person is like God".
Re: your side questions,
Is there already a phrase with this meaning in existence as a motto in Latin?
Maybe that's why I thought of this option in specific.
Second, what are the rules for capitalization in Latin when a phrase is used as a motto? Are all caps acceptable?
All caps are absolutely acceptable. Lowercase letters developed during the history of Latin as a living language. All caps is possibly the most common choice for inscriptions, but you can find different sets of capitalization rules throughout Latin history. If you want to give it an ancient taste you can even replace the u's by v's: QVIS VT DEVS (note that both letter U and the question mark are Medieval Latin).