Historically, you're half right.

The feast of thanksgiving does come from the Anglican calendar of the time, one that got carried along with the Brownists who were early separatists from the C of E. In short, Browne initially was involved in the Puritan movement to reform the C of E, but became disenchanted and became a separatist rather than pursue reform from within. The lasted only for less than a decade in the 1580s with Browne himself, though his movement continued and Brownists were the majority of the Pilgrim group.

So, in Plymouth Colony there was a Harvest Feast Day on their church calendar, and this particular one did see invitees from Indian tribes as friends or at least neighbours. Not "horseshit" as its now fashionable to "re-interpret" history for social reasons, nor after visiting a litany of evil upon them. All the evils you cite really were later events (if memory serves the smallpox in the blankets bit was served up by the French).

That particular feast became an event of convenience when the US decided it would be nice to have a national secular holiday of thanksgiving in the mid 1800s. Many states already had thanksgiving days declared with origins in the church calendar for thanking God for the harvest, and several had early local history tied in, the Pilgrims in Massachusetts, the Jamestown colonists in Virginia for example. I think it was Lincoln who declared a national Day of Thanksgiving and Roosevelt who set the current date scheme (purely for economic reasons).

I don't know who tied the Massachusetts events to the national day rather than, say, the same celebrated by the Jamestown crowd, but it was the event that is most associated, though at the time of that particular feast it was just another harvest day of thanks on a church calendar, nothing special or unusual.