This past Wednesday I arrived at the Apgar Visitor Center at about 0620 to meet a party.
When I walked up to the restroom, there were 3 people standing around at the shuttle stop.
By 0700 there was quite a few people over by the shuttle stop. I think some were Red Bus riders since the pick up is next to the shuttle.
So no, I don't think 0630 is too early.
Two items related to your trip
1) The Xanterra shuttle you plan to catch from St Mary is "first come, first serve".
You can't reserve a seat in advance.
2) Yesterday, a pretty warm day, coming back from ShangriLa/Iceberg Lake, we met two guys with big packs about a mile from Swiftcurrent----at a little after 3pm---headed for ELF
They were hot and sweaty and still had 9 miles to go, including a grind up to the tunnel in that afternoon heat.
These guys looked like very fit "military types" and no "new" gear or clothing.
So there you go!
On a good day like today, you can hike till 9pm with decent light.
9-10pm is a bit risky below tree line(ELF)...imo of course
Cheers and good luck.
You drew a great itinerary.
pete