No point using NXT and the Factory Sealant as they both do the same job, one or the other will suffice, you would be better off with a polish first.
As sealants go the CG Factory Sealant and Megs NXT sealant are both poor IMO compared to other products out there. They both look *ok* but others look great, and neither last much longer than a normal wax, 30 days max which defeats the object of using a sealant.
Good choice on the P21s though.
My advice would be Klasse All in One, Jeffs Acylic Jett Trigger x 2 and P21s
or Autoglym Super Resin, Autoglym Extra Gloss x 3 and P21s