> use tin snips to cut a hole in the side of the metal UPS cover (this was challenging, it was relatively thick metal, I'd recommend using an angle grinder in an appropriately safe environment far away from the internals of the UPS)

Why not just drill a hole? Drill bits large enough to drill a hole for 120A cables exist.

> Get a battery charger that can revive a flat battery, and do a full refresh/renew charge on the car battery once a year or after it's had to push through a power outage that may have used more than a few percent of its capacity.

If you're going this route I'd recommend a marine battery. Car batteries don't handle deep cycles well, and, TBH, UPS chargers aren't designed for failed car batteries (nor marine batteries) and can possibly cause an explosion in the lead-acid battery has a few dead cells.