3 years seems pretty good for using a car battery outside of it's preferred use case.

Better than that :) They were junk car batteries when I got them, and I still got 3 years out of them. But yeah, they didn't get deep cycled much or at all since there were 2 in parallel and the loads I had on them were very small (5-6 500 mAh cell phone chargers and occasionally some LED rope lights).

When I eventually hooked an inverter up to charge my laptop or run a lamp during power outages, they started to show their age with the extra current draw. I think that's what finally did them in.

Yeah, lithium batteries wouldn't like that kind of daily cycle without some kind of charge/discharge limiter to keep them in the 30-70% range. That's basically what hybrid and EV battery managers do to prolong their useful lifespan. I think LiFePO4 lithium batteries would tolerate that better (they're the ones typically used in e-bikes), but they're not cheap. I've also found it difficult / expensive to find solar chargers for them (to be fair, mine is 48v 20AH so finding any aftermarket charger for it has been a challenge lol).