I have a duct into my garage with a fire dampner just as you are wanting. I bought my house this way. My garage is a two stahl tuck under with a tool bench area. I usually have the vent closed off because it really doesn’t do too much. During the winter, my garage stays around 45 degrees all the time with the vent open or shut. I think the heat stays there just because it is a tuck under and gets most of the heat radiating through the walls. I have not had issues with fumes, unless I am working on carberators on the work bench. You can smell a hint of carb cleaner and gas in the bedroom if I am working on a project.
I would say to get a heater just for your garage like those guys were saying.