Is this Latin grammatically correct?

In Latin, the specific construct you want here is called the Ablative of Separation, which is what you use when you wanna say "something (separated X) FROM something else". That actually DOESN'T use a preposition at all (and either way, 'de' is not the right preposition: it means "down from" or "about").

Instead of "Dominatus" (should be "Dominatu"), which more means just "rule", I'd use "Tyrannide" or "Turannide" (same word, basically), which means "cruel, oppressive rule".

For "Freedom", Libertas is fine. So your whole sentence:

Libertas tyrannide

or

Libertas turannide

As for capitalization, most Latin inscriptions are written in 'upper case', mainly because it's easier to carve big, blocky capital letters. There WAS a Latin cursive which served as a lower case, but it was sort of informal. I would avoid capitalizing the first letter of each word, but other than that, do as you wish.