There is no right way, or wrong way to do a cert flight, just opinions.
With that said, I'll give you my "opinion", which we all know what that is worth.
I recommend a 38mm MMT at about 4 lbs. so you can fly all the range of L1 motors, assuming you want to fly this rocket on more than the cert motor.
Based on your "home" field and what the waiver is, go with a small diameter that will get 5,000 ft. or more on large I motors, or a large diameter that will keep altitudes lower. It really depends on what you want to do with this rocket after your cert flight.
KISS is a nice concept, but what do you want to accomplish with your cert flight? Are you comfortable with electronics and altimeters? Building techniques and epoxies? Kits or scratch built? RMS motors or SU? So much of what you want to do is dependent on you. If you lawn dart the rocket and have to build a new rocket and try again, is that OK, or a huge disaster?
You might want to consider the LOC LaserLoc 313 as a good L1 cert rocket. A Fantom 438 EXL would also be a good kit. Both can use DD on the cert flight, or add it in later.
Good luck and use a rocket that will meet the goals you want to accomplish with your cert flight and after.