The whole question doesn’t start. Those rabbonim who assered it started from the premise that it’s a goyisher holiday, but that is simply not true. It’s an America holiday, and from the very beginning Americans have included yidden. Since the first time President Washington proclaimed a day of thanksgiving to the Creator (carefully not specifying any identifying traits that would limit it to one religion) Jewish Americans celebrated it equally with non-Jewish ones. So it’s just as much a yiddisher holiday as a goyisher one.
As for setting up a “moed”, there’s a long Jewish tradition of doing exactly that. Each community has many days that they celebrate or fast every year to remember this or that thing that happened since the time of Chazal. OK, this one does not commemorate any specific event, it’s just to thank Hashem for the harvest; it’s a late Sukkos, adapted for a very different climate and agricultural cycle than Eretz Yisroel. Surely that makes it better, not worse.