Yes, Rot Doctor is good. Realize that this high solvent product soaks in well but doesn't completely embed the wood with resin -- there are plenty of microvoids. I'm not sure that makes a practical difference -- it soaks into all the wood cellulose fiber and shields it. It's so much better having this protection. I like it.
I've used an organic cartridge respirator
for a number of reasons. I just like it better. They are not overly expensive and are comfortable up to a point even in warm sweaty weather. I don't have a beard and the seal is so good you can hardly smell the stuff.
Under any conditions you must have plenty of air exchanges to actually deliver oxygen to you. I'd probably think more of having the fan direct air from elsewhere into your work space and wouldn't worry about pointing it at the work itself. Rot Doctor goes on quickly and you'll be soaking more and more into the surface. On an area like that you'll maybe remix once to make up more. But you'd be there for maybe 5 minutes?
Epoxy is a contact skin sensitizer like poison ivy. You can be sloppy a few times without any effect. And then watch out. You literally won't be able to use un-set epoxy ever again if you sensitize yourself. The solvent in Rot Doctor messes up normal rubber gloves. I used thin utility nitrile gloves. It will mess with them if you get some on them. Have some more gloves at hand to change and wash your hands right after.
There is little advantage to using the more solid boatbuilder epoxies. It really doesn't provide any more security. Moisture control for your shell & bathroom is still #1! Let us know how it turns out.