Ask your self this: What will I gain by doing either of these things? (or alternately, what risks do I face if I do post?)
Sending unsolicited "advice" directly to your former manager will at best be ignored, at worst will be labeled as "sour grapes" and word does get around in cities and industries; people outside the company probably will hear about it eventually.
In my (admittedly limited) experience, even attempting to give constructive feedback in amicable exit interviews usually doesn't result in any change in the company.
You already know that if you post on Glassdoor, you'll be identifiable. See above about how the email would be perceived. Do companies really make changes based on semi-anonymous reviews posted on sites like that? Do you think someone will read that and immediately say "oh wow, he's totally right, we're all fools" and change the company culture?
Right now, you're probably feeling a lot of emotions because your separation from the company was so recent. You want to do something in an attempt to "correct" it. Give yourself time to cool off and I think you'll come to the realization that doing nothing is the best thing you can do for yourself.
If you really need to get it out of your system, write what you want to write, and then throw it away.