WOW; excellent post , GG
Jackson figures are incredibly optimistic:
He assumed Galen never lost even a single page and that he didn't require editing or reviewing even a single word.
Jackson doesn't considered the time required for research and verification; Galen checked on all the available medical and philosophical literature of his time; eg. he quoted many Hippocratical texts written some seven centuries before.
(BTW, I'm sure that by now you're very well aware of this caveat, as Galen didn't have Internet).
But fundamentally, Jackson seems to consider that any successful Roman physician didn't have anything else to do but to write across half a century, being him travelling or taking care of the gladiators; not to talk about experimentation.
Finally, I understand 3,000,000 words is an estimation for what we currently have from him; we have evidence of many other lost works from Galen.