Avi, while I agree there’s no Bal Tosef problem with Thanksgiving, it’s not limited to days on which there is no melachah. If people decided to not say tachanun on Thanksgiving and say Hallel, it would be an issue.
Are there any major groups that actually don’t celebrate Thanksgiving? I’ve always just assumed Chassidim don’t since they have a different approach to chukas hagoyim, but I’ve never asked.
I’m also having a trouble getting a reading on the OP. Is this someone at a point in his life where he wants to proclaim his own gezeiras, or someone who actually poskens like a Rav who assurs Thanksgiving, and he wants to spread his shittah to the masses?