I guess there are multiple comments on these. I would just highlight three.
First and foremost, the resignation is being sent to the immediate manager, who may know the reason, but the same is forwarded to HR later or 2UP manager. Hence, it is recommended that the reason is mentioned in the resignation even though discussed earlier.
Secondly, an employee should give the notice period and commit to handover; the release date has to be decided by the manager. This one looks like forcing upon the manager the release date.
Thirdly, thanking for being given an opportunity to work looks too primitive. I think the employee is there because he deserved it; no one has favored him/her by offering employment.
Hope this helps.
Thanks and Regards,
Sandeep Hariani