Bad 12V battery after 3 years

[Snakebitten]

I grabbed this one

But currently unavailable, so I think this is the current closest same.

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[hatallica]

Doing a bit of a thought experiment ... interested in community feedback.

Using LFP (LiFePO4) battery to replace the Aux AGM: higher capacity; higher resting voltage; concerns with charging at low temperatures (either reduced lifetime or BMS shuts down).

What about a small LFP pack hooked up in parallel and tucked in to the extra space of the battery compartment? Here is what I am thinking:
Battery pack: realistically about 5Ah
BMS: documentation sucks, but seems to have charging protection under -5C.
A bit of soldering, a bit of shrink wrap, and short wires with battery terminal lugs. I already have the tools to balance cells. All in for well under $50, but does it actually gain me much?

Normal temperatures: nearly double capacity and higher resting voltage.
Low temperatures: LFP stops charging until the cabin warms up a bit (may be a while depending on the thermal mass of the LFP and the hysteresis of the BMS). LFP continues to discharge in parallel with the Aux AGM. The Aux AGM behaves normally, but is starting from a higher state of charge.

Not sure if I want to go down this route yet. What are some obvious gotchas?