| | Re: stem plants that thrive in hard, alkaline water
Hmm. Anubias might work for you. Probably java fern too.
8.4 is pretty high for freshwater. Why is it that high? Perhaps the soil you used is causing the problem, or maybe an ornamental rock. pH 8.4 is roughly what the ocean is. Freshwater usually never gets this high without a substantial amount of minerals being added.
Even so, I'm not sure that the pH you have would necessarily kill all the plants you have owned. I think perhaps lighting might be an issue or even the crayfish.