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And Model? - Part Number ? Have you measured the resistance/continuity of the heaters and confirmed voltage across them?

TOPDON is what I'd consider middle range - there's a few other brands that are cheaper and have good reviews.

The Fluke 77 was the top of the line multimeter when I got it in 1983 and it's still going strong, however that is not what I use to assess a battery's health for the reasons explained above.

A 600 CCA rated battery would be consider past its useful life long before the CCA drops anywhere as low as 10.0A The settled voltage of the battery is only an indication of its level of charge, not an indication of its health/CCA capacity. .

A volt meter is not a reliable test of a batteries health - its CCA ability.

Measure continuity of both O2 sensors heaters and if that's good confirm voltage across the heaters. What brand and model O2 sensors did you install?

Glad you got it resolved. I'm a puzzled by that comment - based on your own description the signs were there of a bad battery, and that's what I based my recommendations on. For the benefit of others, most auto/parts/accessory stores have test equipment that can reliably diagnose a bad battery, or even a battery is approaching end of life long before it presents a problem - they can also supply and fit a battery equal or better than the OEM at a price significantly lower than the dealership. Good quality testers can be bought on-line at a reasonable price and are real simple to use.

The tester that auto shops and many DIY'ers use does not require more skills than basic reading. TOPDON BT100 Car Battery Tester 12V Load Tester, 100-2000 CCA Automotive Alternator Tester Digital Auto Battery Analyzer Charging Cranking System Tester for Car Truck Motorcycle

Any explanation without testing would be a thumb-suck. Testing is what's needed, and the "Autozone" testing suggestion was meant to be an example of the service such stores offer, and most likely some in Canada do the same. A dead/discharged battery could be the symptom of a failed/failing battery or a parasitic drain or a charging system failure.

Get the battery and charging system checked at a place like Autozone - typically free of charge for the test.

Keep it to one thread. Use an OBDII dongle and Forscan to scan for codes.

No idea, never used a torque wrench on that. Turn it down until it bottoms out and that's it - its seals on the o-rings and doesn't need any torque for that.