PPS into GPIO will get you sub-millisecond error levels - you've got interrupts, after all! Sub-millisecond jitter isn't perfect, but it should lock quite happ

PPS into GPIO will get you sub-millisecond error levels - you've got interrupts, after all! Sub-millisecond jitter isn't perfect, but it should lock quite happily, you can get a lock on time sources with orders of magnitude more jitter.