Whoever told you that higher levels need more severe bits was full of it!!
I ride my Morgan in an eggbutt snaffle and my Arab in her halter alone,
even on her first two 30 mile rides this summer. You are right, good
horsemanship should be able to succeed where a severe bit fails. One
option is to ride without using the bit unless you have to, relying on seat
and legs. Dressage lessons are a very good step in this direction.
However, don't put your horse in a stressful situation (like the shotgun
start at an endurance ride) in a less than adequate bit until he has proven
his willingness to obey. My friend starts a ride in a bit, then takes it
off at the first vet check, when her horse remembers her training and has
her mind on the job.
Shannon Loomis and Quail Meadow Star