Regarding forcing the device to SF12, see this (and plenty other threads on this forum) about it:
Assuming ADR is on:
Before joining, forcing the device to SF7/DR5 is perfect. If it is close to a gateway the device will remain on SF7. If the device is not in range of a gateway using SF7, the module will slowly switch to a slower SF until the network hears and responds to it.
ADR off:
For mobile devices long airtimes are actually worse than short time on airs. There are many measurements made with TTN Mapper to prove this. So using SF7 is anyway better.