I frequently fly at 10 to 12 thousand feet msl and have no issues. Supplemental O2 is only required from 12500 to 14000 for durations longer than 30 minutes. It is required above 14k for the crew according to the regs. At 15k, everyone on board needs oxygen or a pressurized cabin.

Tiger got to hunt, bird got to fly; Man got to sit and wonder ‘why, why, why?’ Tiger got to sleep, bird got to land; Man got to tell himself he understand

Done, and link forwarded to my non Lemmy using selfhosting mentor. Maybe can get at least one more respondant. Maybe more. Good luck!