I doubt anyone can give an authoritative answer other than: no, always using SF11 or SF12 is simply not allowed. Period.
On the other hand, with the ideas you already described yourself, you won’t be always using SF11 or SF12?
Circumventing this by randomly using, say, SF7 every now and then, while you don’t expect that to be received at all, may feel like just causing more traffic. But as SF12 uses about 25 times more resources for the radio than SF7 (it depends a bit on the application payload size), both in terms of eating your maximum daily airtime and consuming your battery (for the radio part), you may still benefit from trying to use better data rates:
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Using so-called Blind ADR may save both airtime and battery. Like:
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A single message on SF12 uses almost the same airtime (hence battery) as the total airtime for 4 separate messages on SF8, SF9, SF10 and SF11.
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When not using SF11 either, instead of sending a single message on SF12, you could send a total of 6 when using 2 × SF8, 2 × SF9 and 2 × SF10.
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Sacrificing one of the SF10 transmissions allows for, e.g., 4 × SF8, 3 × SF9 and 1 × SF10, their total airtime still being the same as a single transmission on SF12. Are you seeing a pattern?
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According to Max range and performance within limitations the range won’t double while
doublingeven quadrupling the cost for time on air when using SF12 rather than SF10: -
The longer the time on air, the larger the chances for interference; see Is there a correlation between payload length and potential packet loss?
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Not my cup of tea, but using SF12 for devices that are moving while transmitting, may also not be as good as you may expect:
So, the SF11-SF12 rule is easily circumvented, with possibly great benefits. Also, of course, other radio technologies may be a better fit for vehicle tracking.
(Please post any responses/queries regarding the quotes above to those existing topics.)