I’m assuming you are referring to the widely discussed (before hand) changes that were being made to templates? If not, please correct me. Changes to templates don’t require you to “completely rebuild my ha instance”, they require you to amend your templates.
That means nothing. The card may be a dud, corrupted flash, who knows, you may have even purchased a fake/copy. Don’t assume the card isn’t at fault when that is almost always the most likely point of failure on a Pi build.
I was to know this how? You posted no system information. Memory usage can spike and cause freezes and HA to stop, I experienced this exact issue myself some time ago while still using a Pi3 in my business. With no config changes whatsoever, I moved to an old Dell USFF machine, restored my snapshot and haven’t had an issue since. Don’t write off the possibility that it’s not the Pi3 unless you have tried something else to rule it out.
Foolish how? Unless you have dozens of ports opened up to the internet and don’t use SSL, then yes, you would be right, that would be foolish.