First, a nitpick: AGM is a type of lead acid battery.
I put in a LiFePO4 starter battery in my truck a few months ago. Too soon to say whether I’d recommend it. I wouldn’t
not recommend it so far, but longevity remains to be seen.
My reason for going lithium is to power my camping/overlanding/glamping stuff. The replacement battery is dual purpose: starter and deep cycle. I can charge the battery both from the truck (eTorque, so DC-DC rather than alternator) as well as solar panels; I have 350W on my rooftop tent.
Biggest disappointment so far is that I haven’t been able to get the truck to charge the battery to the levels I want; it usually settles around 13.2V, which is around 70% charge. That’s fine for normal usage, but if I’m pulling into camp after sunset, pretty low (although still more than enough to power my fridge, heated mattress pad, etc. through the night).
Can you run a lithium starter battery? Absolutely. Should you? Jury is still out.