As per TJ states, keep it as simple as possible.
It’s a resignation letter, all you need in it is written intention to resign. Make sure the letter is dated and the date of your last day of work stated (if you can work it out).
Send a copy to your line manager and HR person (if there is one) or whoever deals with HR and keep a copy yourself.
There’s no need to thank people, or raise issues in it, all that’s needed is to say you are resigning so there’s proper formal written evidence of that fact.