This is the 1595th most frequent Japanese word.
尽くし
"尽くし" means "exhausting," "doing to the utmost," or "a collection or variety of things."
Here, '尽くし' (tsukushi) implies 'full of' or an abundance, used to indicate something is replete with a certain characteristic or element (traditions in this case).
日本文化は伝統尽くしです。
Japanese culture is full of traditions.
In this usage, '尽くし' describes a situation being abundant in a particular negative aspect (problems), emphasizing a challenging experience.
彼は問題尽くしの一日を過ごした。
He had a day full of problems.
In this instance, '尽くし' conveys the idea that gratitude prominently fills the letter, suggesting an overwhelming focus on expressing thanks.