I found a different opinion of when the letter to Wythe was written, namely March or April 1776. The following is from an editorial note on “Thoughts on Government” on the National Archives website (https://founders.archives.gov/documents/Adams/06-04-02-0026-0001):
“The italicized clause indicates that Adams had not yet written his plan of government. Yet, on 27 March, Hooper and Penn left Philadelphia to attend the North Carolina convention, so that, given the account of events that Adams wrote to Warren on 20 April, Adams must have written out his scheme for Hooper and Penn before they left the city. That fact would date the earliest versions of the Thoughts between 19 and 27 March. The Wythe version may well have been composed soon after 27 March, for Wythe saw either the Hooper or the Penn version before the men left Philadelphia; the Sergeant version followed, completed at the latest by 15 April, when Adams told Abigail he had used up ten sheets of paper. We know that on 11 April, Sergeant requested that Adams send to him a ‘Copy of a Paper I spoke with You about the Evening before I left Town,’ which was very likely one of the first three versions of Thoughts on Government.”