What Rickair told you is correct. The easiest way to get from your MULTI CFI/CFII to ASEL CFI/CFII is to do the ASEL ATP add on check and then the ASEL CFI add on. Unfortunately, you may run into DPEs who don’t understand the rules and insist you can’t go straight to the ATP add on and must do the ASEL commercial add on instead. I ran into this myself. The commercial ACS will force you to do a few more maneuvers (power off 180, 8s around pylons, etc.) but is relatively low threat. Read the ACS and do a couple rides with a civilian CFI to make sure you meet required standards before the checkride. Same with the ASEL CFI add on.