In my opinion, formal salutations like this do not need to be translated into a formal English business letter, and should be left out entirely.

As in any language, business letters have certain standardized formats dictating the order and location of content. In English, salutations and words of thanks come at the end and are usually short, with the objective of the letter written concisely at the beginning.

Consequently, when translating business letters, I think it is always best to omit these phrases and things like 季節の挨拶 entirely.

Good luck.