Update 10/15/20
@svreef @Patio After our conversation yesterday, I went and purchased a bottle of Instant Ocean's BioSpira and added it to the tank. It's not conclusive, but I'm inclined to believe Microbacter 7's marketing is deceptive re: it starting a cycle.
I started my cycle on 10/9, and I've been checking ammonia and nitrite levels once daily. Ammonia has gone down slightly (2 > 1.0-ish) between 10/9 and 10/14, but no detectable nitrites. I checked nitrites yesterday evening at ~5 PM, still not detectable. I added the bottle of BioSpira immediately afterwards.
At about 11 PM, I decided to check nitrites again before bed, and saw some movement: not at the 0.25 ppm mark, but a definite slight purplish tinge to the test (I use the API Saltwater Master kit for cycling). This morning I checked again, and...well, I think I have nitrites now.
Ammonia also dropped from ~1 to ~0.5 ppm overnight.
I will not say BioSpira is definitely the cause, because correlation =/= causation and ammonia was starting to get close to the range I saw the nitrite spike in my first tank. However, this strongly echoes the cycle of my first 20 gallon tank where there was four days of minimal movement from ammonia/no nitrite with MB7, then I added BioSpira on day 4 and saw a rapid nitrite spike.
Lesson learned; I'll be going with non-MB7 cycling products from now on.