Is this a good mask for sawdust? Is it comfortable to wear for a long period (8+ hours)?

After some time you will want to take it off just to give the area of skin that make the seal some air and then you are good to go. The longest that I have use mine for is about 7 hours. I took it off when I took a break and was good to go some more. I never had to take a break because the mask. And yes, this will work on sawdust, but it is a bit overkill. All you need at least is a P95 mask and you will be good to go. But remember you need at least a P95 do not use an N95.

Yes. This would work for your application. All you have to do is purchase the correct filter for saw dust. I personally only worn mine for about 2-hours. It was very comfortable, cool and easy to put on and take off. I purchased filters for chemical vapors and P100 particles etc., and were more expensive. I can only imagine that they would be far less expensive for dust. It's worth looking into, though.