This is an update to the above post from November:
I replaced the booster with a used one and brakes now feel great. Firm pedal and what I would call normal behavior.
I have the exact same issue and have done exactly the same things you have done trying to remedy the situation. My son, who was employed for a while as a Toyota mechanic (now an electrical engineer) feels that I have a vacuum leak on the firewall side of the brake booster diaphragm. I've done a...