I've experienced insomnia after stopping meth, although I usually end up sleeping a lot once I manage to enter sleep-land (not uncommon for me to wake up at 12 - 1pm). I don't think it's that uncommon, probably has to do with our circadian rhythm being somewhat fucked, but it doesn't last too long if you practice good sleep hygiene.
IME the worst thing is the lethargic feeling it gives you and the complete lack of motivation. This can be partly prevented (not 100% though) by not binging for multiple days without sleeping, I've said it plenty of times but prolonged sleep deprivation is one of the worst things you can do to your brain&body.
You quit the "several other drugs" at the same time? If so that might be contributing to the insomnia, THC will definitely do that.
Also the benzos you're taking are probably preventing the worst of the crash as well. As you know, flualprazolam is a very potent benzo so it's no wonder you're not getting much anxiety. Be careful about the rebound though...