First of all, it depends on the number of words required for your thesis. If it is only a small paper of 5,000 to 8,000 words, then the topic of "national character" is too big. There are many aspects of national character, and it is enough to pick out one of them. If it is more than 10000 words, you can write two or more aspects according to the collected proverbs and related materials.

As for the train of thought, I first put forward my own argument that "proverbs can reflect the national character of the Japanese", then write some proverbs about how to reflect the national character of the Japanese, and finally summarize and return to the argument to make the end.

In the middle part, proverbs can be put forward first, and if possible, the source of proverbs can be added to analyze from which aspect and how proverbs reflect the national character, whether this national character is beneficial or harmful, and how it affects Japan now.

That's all I can think of, for reference only.