I've finially received my new batteries. Thanks to @neftaly who has a family hookup to get things imported very cheaply from china, I got 200 AH LiFePo4 for $340 NZD. That is about half the price a lead acid, plus you can discharge these batteries deeper than deep cycle, so in practical terms (not marketing) they actually hold more usable energy.
@neftaly gave me some advice for managing the batteries charge manually, reposted here with his permission:
I took my BMS out entirely for now, going to just manually maintain them until I get my new one. As long as you never let any individual cell go over 3.65v (and avoid discharging to under 2.8v, which isn't as bad as overcharging but still bad), you should be golden. Run a multimeter over each cell and try and keep the voltages the same (you can do this while they're still connected together in series). Do you have the ability to limit your charger to 14v? Also a cut-off switch for your alternator would be helpful.
They will mostly self balance as you take power from all of them when in series (higher voltage cells will have more power taken from them, so will drain faster, and return to equilibrium). But if one gets out of whack, you can just put a load on it to drain power, such as a 12v incandescent bulb across the terminals, until the voltage drops enough.
If one gets super discharged, you can try disconnecting everything, then reconnecting everything together in parallel (all positives connected together, all negatives connected together, nothing else connected) and leaving it for a while. Power levels will gradually equalize, though it gets exponentially slower as the voltages get closer together.
Be very careful when you're installing the battery bus bars and stuff - it's sooo easy to accidentally short circuit it when you're using a spanner. I wrapped mine in electrical tape to help.
The first time you connect solar etc, keep an eye on it as voltage hits the limit (use a multimeter, don't just trust the votlage readout). You wanna make sure your solar charger cutoff actually works as expected.