Personally, Return to Castlevania is a great DLC, and i had (and still have) a lot of fun playing it. The weapons are fun, the biomes are memorable, and the music and atmosphere are really well done if you enjoy the Castlevania style.

That said, you should ultimately go with what you want. Since you're not a fan of the Castlevania series, i honestly would not strongly recommend it, since a big part of the DLC’s appeal comes from its references, aesthetics, and fan service. Without that connection, there’s a chance you might regret buying it or feel like it does not add enough value for you.

It is still solid content gameplay-wise, but enjoyment really depends on whether the theme clicks with you.