Arlo doorbell sensitivity is terrible
Your mind is made up so nothing I say will change it. All I can say is that with a zone to exclude the street and a motion setting of 25 in my modes and rules I get almost no false triggers while still getting people walking up the walk ~20-25 feet away. There's been no need to change motion sensitivity due to temperature although I could see some corner cases where that may be needed.
As noted in documentation as well as the notice of a trial subscription on the box, a subscription is needed. This has been true for a couple of years now, starting with the Ultra (anything newer than the Pro 2).
And I'm retired IT for an ASIC design house (previously an R&D electronics tech) so I can figger things out, too.
I would submit that it's not temperature variance causing the triggers but the change in the sun's angle causing more IR from the sun being reflected off passing cars. Also, you never said whether you're reducing sensitivity in your modes and rules. As for zones, other Arlo cameras allow zones to be used without a subscription as long as they were powered so I would ass-u-me that the doorbell follows this allowance.
And it's not "my" sensitivity. This is a user forum and I have no official connection with Arlo or any other company.