Masteries wise, would the cooldown on bulwark be more beneficial then honor guard? Or juggernaut vs Mastermind, you may not get that 3% extra health, but having more opportunities to use exhaust/ heal/ clarity could be more beneficial.

I've been playing galio for a few weeks now and think I've gotten him down for the most part. Tend to build Chalice of harmony, followed by basic boots. Then I go for heart of gold, Force of nature if ap heavy team, thornmail if AD, or both for the health regen from Force of nature. Finish up Randuins omen, and then insert other tank item here. Have had alot of success with the build, pulling off 41 assists in a game XD

I like Mastermind, I like Intelligence, I absolutely hate having to spend 17 other masteries in the Utility tree in order to get them.

The setup I run, currently, is 1/17/12, taking Enlightenment, Vigor, Greed, and the obvious. That's for any support, mind you, and it's been working well for me.

You need your Philo Stone and or HoG first, getting them at 10 minutes instead of 5 is a lot of gold you aren't getting.
Otherwise it sounds like you are fine. might want to consider Sunfire in addition to/instead of thornmail. It combos with your ult very nicely

Provides more AoE damage
More health and Armor allows you to take the dmg during your AoE better.