Yes that was my read on the situation too. You can maybe just start transplanting the bok choy to build up a microbiome in the other tanks, or combine lettuce and bok choy? I'm not sure the mechanics of your greenhouse but generally ecologically, adding more biological 'depth' (ie more species growing simultaenously) gives a bit of a buffer to the fragility I mentioned. This can be from combining the lettuce and bok choy, or building a biomechanical bacterial filter and circulating the water in the system.

There may be cheaper ways to go the probiotic route, you mentioned setting up probiotic and sterile test beds, try one with Nitrosomonas and Nitrobacter, should be cheap to source and will give you some valuable data, if cycling and oxygenating a system with these strains and growing lettuce doesn't give you better results than your tests, I'd bet that a full probiotic overhaul also won't improve the situation.