Really struggling with the meaning of this sentence from ちびまる子ちゃん:

まさにうららかとしか言いようのない日だね

My rough translation effort

Surely, beautiful is the only way to describe this day

But I'm struggling to break down the grammar here, in particular the としか part.

My guess it that うららか is an onamatopeic kind of word and they seem to like to take と and that is followed by しか meaning "only". So I would guess at "Surely - beautiful - with the exception of - indescribable - day - is". Or in other words "There's surely no way to describe the day other than beautiful".

Am I even close to a correct understanding here? If not please correct me. Many thanks.