Yes, I think 「安らかに眠れ」sounds natural. However, it also sounds "rough" and "rude" if you say it to someone you're not close. 安らかに眠って下さい is more commonly used. I hope this helps :)

多分「安らかに眠れ」が一番いい訳だと思います。 Note: while "Rest" in English is used in various settings, 「眠れ」 is not. It is a ‘command form’. When I hear 「眠れ」, I imagine someone saying that to themselves or something that is written, as akatuy san pointed out. hth

私には翻訳調に聞こえます。　洋画の題名にでもありそうな雰囲気、あるいは三流小説で使われそう。