+2 for flanking in PF1 seems too low to me. You are attacking someone who can’t even see you to react. The only thing in your way is his armor. I would opt for +4 in this scenario. At the very least a +3.
That said, I think that the typical rogue backstab bonus in D&D is +4? So maybe that should be increased too, especially in PF where all the numbers are higher.
I need to look into this across all editions. Not making any changes for now, but this needs looking into.