Since even the spacing between the tiles is uneven Grid by Cell Size is not an option as it would cut up the tiles at random places. I've realized that there is an enormous amount of manual work needed to get these tiles in a workable shape, since I literally have to cut out each tile and put them in a proper tilesheet myself.
I think putting more thought into the usability of the tiles would up the quality of these sets but I'm not going to refund it since I really like the aesthetics, and will make it work one way or the other.