Is it "I am writing this letter, I write or my reasons for writing this letter"?

You need not say either. It is obvious you are indeed “writing the letter.” You need not say: “I am writing for these reasons.”

Dear XYZ,

I am concerned about ( state your issue)

or

I would like to suggest (details here).

——-

Don’t waste time telling the reader that you are writing to tell him/her something. Just get right to it.