I’ve always brewed indoors in my shop, it’s nice to have heating and cooling. CO shouldn’t be an issue if your burner is properly adjusted, meaning a nice blue flame, a CO detector is still a must though. A small fan in a nearby window would be enough ventilation. One concern is bringing full tanks of propane in to a heated place from the cold outdoors, if they happened to be over filled there’s a potential that the relief valve could open, not good. I would look into using the household gas supply if you have it.
That steam should be vented, too. Brewing next to an open window with some cross ventilation will help direct both the CO and the moist air, but you may need a fan to move the air out better.
Right, move indoors to the garage. Lay some old carpet/cardboard on the concrete to keep your feet warmer. Do your mashing inside the house. Drain into a pail and carry out to the garage.
I used to brew outside, when it was cold I would brew in garage and leave two windows open a little and the garage doors up a foot or so. Gave some good ventilation without totally freezing my butt off.
I do now brew inside my basement with natural gas and good ventilation.
However, I would never, ever brew in my basement with a propane tank. There are so many bad things that could come of this.