Old EV batteries are great for energy storage. A worse weight-to-capacity ratio doesn't matter for batteries sitting on the ground. A battery that holds only 70% of its original capacity is considered worn-out for EVs (and even replaced under warranty), but grid storage isn't driving anywhere, so any capacity left is still useful.
Parting out two or three dead battery packs to cull the best of the survivors can improve things quite a bit. And as you say, on a stationary pack you can afford to overdo telemetry, cooling, and safety circuitry because it doesn’t have to move, let alone accelerate.
I don’t know what the half life is like for the reused cells though. Do the cells that lasted twice as long as their neighbors continue to outperform or do they revert to the mean over time? I could see either being true. The days when you accidentally produce cells that are several stddev better than your target quality should make cells that last longer, unless they’re sold to a leadfootted driver.
I have been out of the battery tech game for a while now but decades ago we were balancing individual NiCd and NiMH cells for optimal performance, is this basically the same thing?
https://electronics.stackexchange.com/questions/463591/nicke...
In a parallel bank, a single cell going bad can bring down the rest to the same voltage. Even worse, if the bank is directly connected to other banks it can take out them as well. Also, if there is an internal short in one battery, the rest will pump current through it very effectively lighting it on fire. Individual battery protection circuits, smart switches, and internal short detection can help with this.