BS”D
The problem is that while the motivation is sincere, I don’t see the chumra because it is based on a mistaken understanding of how cream of tartar is made.
Seems to me that there is no such thing as commercial non-kosher cream of tartar. Only way you could get non-kosher cream of tartar is from a non-Jewish amateur winemaker who improperly makes his own cream of tartar and then the stuff probably wouldn’t work for making play-dough or icing or anything else.