So I think this matches what you are saying: explicit vs. implicit. That website says "the radio transmitter will add another 4.25 symbols resulting in a final preamble length of 8 + 4.25 = 12.25 symbols." I think a preamble setting of 8 means that there are 8 up chirps and 2.25 down chirps. Where are the other 2 symbols coming from and what is their purpose?
I think that the best strategy may be to approach a "group" of 8 devices on a lower bandwidth and higher spreading factor (explicit mode) that would support the long distance "roll call" requirements. I could then jump into implicit mode by "logging in" to the groups that would set the TX and RXs to SF=6 and BW=500000, gain the benefit of a higher symbol rate, accept the short distance communication, perhaps drop preamble. Then I would need to figure out a way to either log out or time out the RXs that would reset them back to explicit mode long range. Log out could be an implicit data receipt of a number not designated such as 000000 or 111111. What do you think?