Thanks for the comments everyone...to address some of the points:
So... how do you get a perfect 1" hole into your bucket?
I traced a US quarter and used the tip of a small kitchen knife (couldn't find my xacto) to cut the circle in multiple passes. Then I smoothed out the edges with a bit of sandpaper.
I'm more curious how it gets a solid seal with the curved surface of the bucket.
I was worried this would be a problem, but apparently home brewers use these on buckets all the time. The spigot has two rubber washers to seal over any rough edges, and the bucket flexes a little as well.
How do you lift the bucket above the tank?
Proper body mechanics! Helps that I only need 2 or 3 bucket fills per water change...
I guess you could sit the bucket on a corner of the tank for it to drain into the tank, but that seems to be a disaster waiting to happen.
I cut a small piece of plywood to fit on the corner of the tank, and set the bucket on that while it drains unattended. The bucket would have to be pretty far off-center for it to be at any risk of tipping.
Water would come out slower from the spout, taking longer to fill.
I asked on Amazon about some other spouts, and people responded saying that it would basically take way too long to drain 5 gallons. Those were "bottling spigots" (filling beer bottles). This one has a larger capacity and, while I haven't timed it, I'd guess that it can easily drain 5 gallons in less than a minute.
If I open it full-bore and aim straight down, it digs a hole in the gravel through a foot of water.