Like you, I have doubts the anthrax attacks were carried out by the fanatically religious al-Qaeda (though I suppose one could not rule out a secular "surrogate".)
My opinion is based,in part, on the notes enclosed in the letters.Putting "Allah is great" at the bottom of the note would be unthinkable to one who was devout.
A note written by a devout terrorist would start out with something like: "In the name of Allah the merciful and compassionate..."
I do actually think that "Islamists" at least determined the timing of 9/11 and the initial anthrax mailing. The conventional story is that Atta chose the date and then phoned Hamburg to keep them in the loop, and I've noted before that 9/11 is the Egyptian Christian new year, while 9/18 (when the first anthrax letter was mailed) was the Jewish new year in 2001. It seems like a variation on the abortive "millennium plot" of January 2000.