You will have to book through the Fantasmic link on the same page as the rest of the DHS reservations if your goal is to get VIP Fantasmic seating. They are pretty limited, so you may not get anything to come up that you want for a while- and the odds to get a Minnie Holiday and Dine Fantasmic are even lower than a normal Fantasmic package. At one point Dec 7th was a day we were looking at to tour DHS, but we couldn’t get any Fantasmic dining options to come up that we wanted- so we moved our touring day to a another slow park day later that week. Keep checking though, one might come up for you.
There is no way with a 7:45 ADR that you could get there, wait to be seated, order, eat- then make an 8:30 show. There is already a long line to get into Fantasmic by 7:45 since it starts at 8:30. My guess is the one you obtained popped up because some other family opted to do Fantasmic instead of the character dinner. Fantasmic dinner packages are earlier reservations.
Minnie’s Holiday Dine as a stand alone reservation is pretty hard to get as well- so you have a tough choice to make. You were lucky to get one this late in the game, so I would hold onto your dinner ressie until one comes up that you can cancel and rebook for. Cancellations happen all the time- so with some persistence hopefully something will shake loose for you.