As your eyes are pretty close in refraction, I would expect no need for mixing and matching if you have the patience for the weaker eye to catch up each time. The only hesitation I would have is about the choice of frames and getting the PD right. If you have the PD noted on the prescription for your current glasses, that should be fine. But pupillary height can also be an issue, and this is a bit of a gamble, as one cannot specify this on a Zenni prescription. A good optician marks the pupillary height as well on the instructions to the lenscutter.The price of Zennis goes up with higher diopter lenses, so if you get this wrong, you may waste a lot of money.
I see that you are going for .50 reductions until you get to -2. This is rather ambitious, but on the other hand, you can always order more in between glasses if you find this to be the case.
My advice is to order only one pair of norms and one pair of diffs to start off with - there will be other sale offers in the future. I ordered too far ahead, and regretted it. I discovered, when I started taking lenses out of the frames, that the PD for the two eyes is not the same, that the left eye has a different pupillary height than the right, that the axis of astigmatism is variable in the left eye, that I overestimated the potential rate of reduction of astigmatism in the left eye relative to the reduction in spherical, and that the right eye improved much faster than the left = 14 pairs of pretty useless glasses, in spite of being able to mix and match. This is not a great disaster for me, as my much stronger right eye has allowed me to be uncorrected most of the time, and I have not had to buy replacement glasses. But with your degree of myopia you may end up having to replace glasses - or you might just get lucky on the fit of the frame, the PD and the PH.
Depending on where you live, you might also run into trouble with import tax on such a big order.