The third time was, in fact, not the charm. The quality of this new (admittedly cheap) lens is terrible lol
I feel like I either need to suck it up and spend the money to buy a high quality Canon lens, or just cut my losses on this idea and use a short focal length scope to make a mosaic. The latter option is likely the better one — I don't see any reason why it wouldn't work, even if it's not as elegant of a solution.