The problem people generally have going into the regionals is the 121 training environment. You don’t need the experience in a jet, high altitude ops, etc. to get through 121 training. You build experience as an FO (CA in training) on the line. As a CFI (a CFI that takes their job seriously), your head is always geared towards learning, even while you teach. That discipline and motivation is all you need to pass 121 training. The guys that come straight from 135 generally have issues with training because their head has been out of a “schoolhouse” for so long. If you do go CFI and don’t get your CFII you’re doing it all wrong.