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Perhaps I missed it:
What is the battery voltage when the engine cranks slowly? For that matter, what is the battery voltage with no load? A fully-charged "12-volt" battery had better have 12.6--12.7 volts with no load, and at least 9.5 when cranking, but more likely ~11 volts. More is better.
From what I'm reading, you've gone to heroic lengths, doing everything except verifying one of the MOST COMMON sources of cranking problems--a discharged or failing battery. At minimum, you need to check open-circuit voltage and cranking voltage. Ideally, you'd have the battery properly load-tested.Welp....both cold and hot starts sounded strained this morning. But it is 24° outside. Time to get the battery checked again.
Safe to assume that months of heat soak starts can damage my battery's performance?
Well it's been in the 30s here [emoji28]At minimum, battery is discharged. (I'm taking a wild guess that it's just plain defective.)
Slow-charge overnight, re-test. Disconnect charger, let battery sit for an hour.
A fully-charged battery should register 12.6--12.7 volts. When cranking, voltage shouldn't be under 9 V, more is better. Battery voltage does depend on temperature, if the battery is cold you'll have to warm it or compensate your voltage readings according to published volts-vs.-temp charts.