Nytrunner
Pop lugs, not drugs
It's my understanding that the 5:1 rule is, in fact, based on initial thrust not average thrust. The problem with this kit seems to be though that even the recommended initial thrust is about half of what it should be to get adequate velocity off the rail. It is a pretty big rocket for the motor class just in terms of diameter (3") and length (5 ft) so maybe it's just generating a ton of drag relative to its weight? idk
I've heard several RSO's base their judgments off of average thrust unfortunately.
I think you're asking good questions. This is your rocket, and if you don't feel comfortable with a minimum 5:1, then by all means, choose a motor that simulations show will clear the rail at a sufficient speed. Also, a 5' rail sounds extremely short, so THat could explain why you're having trouble getting up to speed. Have you asked the club you're going to fly with how long their rails are?
Just for anecdote sake: My L1 was paper/ply, 4" by 52", weighed 5.2# lbs loaded (It's undergone weightloss surgery since then), and flew off an 8' rail using an H120. Perfect flight to ~1/4 mile. The RSO raised an eyebrow at the H120 on a 5 lb rocket, but he accepted the knowledge that an H120 has initial thrust ~145N and didn't predict any issue. (Which he was right about )