Oh believe me, I massively advocate for bottled bacteria products (in general) lol. I got my PhD in microbial physiology, and reading up on nitrifiers and their resilience wasn't difficult for me. Turns out they are super resilient, able to survive ammonia and/or oxygen starvation conditions for prolonged periods before going dormant, let alone die off. So absolutely no reason why they can't survive in a bottle for a long while, and even if there is die off, there'd probably still be enough individual living cells for the product even after a year on the shelf. Of course, just because that's possible, does not mean all products are created equal. Some products are straight up not even suitable for cycling, like
Seachem Stability.
But yeah, Fritz make products that work, and work well. Same with
Tetra SafeStart (allegedly, since I have never used this product myself, but have also seen plenty of reports that even after ruling out potential ones with confounding factors, seem like it works well).
However, while FritzZyme TurboStart 700 works well - from all the reports of its usage it seems like it cycles the tank within a week 95% of the time, and in fact, instantly 5% or so of the time - amazing, FritzZyme 7 from reports I have seen, and from my own experience, takes much longer to start working. Mind you, it definitely works, and definitely speed up cycling by a lot, but yeah, still nonetheless takes time.
pH can naturally lower due to various reasons, though going from around 7 to 6 is quite drastic. My recommendation is to get something to buffer the pH, like
crushed coral. That would get pH to get to around 7.6ish eventually (and keep it around there). Better for FritzZyme 7 nitrifiers anyways - apparently their optimal pH is between 7.3-8.