It’s always 4/5; see Payload with only 2 characters and coding rate 4/7? - #14 by arjanvanb
And note: SF12 is only allowed in combination with ADR.
Minutes?! How often are you sending? Beware that the TTN Fair Access Policy allows for at most 30 seconds time on air per device, per 24 hours. So, an airtime of 2,793.5 milliseconds for 64 bytes on SF12BW125 imposes a limit of 10.7 messages per day.
When transmitting all day, on average this needs a minimum of 134.1 minutes between the starts of two uplinks, or a maximum of 0.4 messages per hour.