A friend of ours has a 6/7 yo child who is very axious, etc. to an extreme about dental procedures. Her dentist gives a script for Valium (4 tablets!) to be taken ahead of time, then uses nitrous, etc for the procedure.
We have a pediatric dentist who specializes in special needs children--CP, autism, etc; those with oral sensitivities; severe anxieties, things like that. You would not believe how well kids respond to him! I believe he does conscious sedation; many of his procedures are done in the hospital with an anesthesiologist. There is a way, don't remember the specifics at the moent, to "layer" the meds so that thye start with something oral, then when they are calm and almost out, switch to the more heavy meds. Is your dentist familiar with working with kids? They should know how to do this...
You may also want to petition your dental insurance for an out of network approval because of extenuating medical circumstances if you can find another dentist, especially if you have a diagnosis. A pediatric dentist, esp one who specializes in difficult situations, sounds right up your alley. Are you by chance near Dallas/Fort Worth?