Suggestions for off-grid back-up system

[Hedges]

Which does not mean that a bigger outage cannot happen.
When did your house burn away last time?
A good size automotive battery is ~ 1KWh.
Nominal. Expect realistically 600Wh.
How long will you run your house on that?

With between 600W and 1000W of PV panels, I expect the 3000 Wh draw of refrigerator & freezer to be met every sunny day for half of the year.
With car alternator available to make up the shortfall for overcast and winter days, those are taken care of too.
(best if you have two cars, can jump-start yourself in the event you get battery run down. Some inverters are designed to disconnect before so low you can't start.)
I think this would take care of the occasional power failure OP was concerned about.
He's not trying to run the house, and refrigeration should be powered direct-PV, not from battery.