Scott, I think 205 is actually a faster hardener, so switching to 205 wouldn't be a good idea in this case.
When I'm mixing up a large batch of West System epoxy, I find it helps to mix it quickly in a cup and then immediately get it spread out over a larger surface. This keeps it from getting hot and gives you a lot more working time. Depending on the application, sometimes I dump it out on the part and get it spread out roughly, then come back and spend more time with the squeegee (aka old credit card) moving it around to the final thickness desired. Alternatively, I sometimes dump it on a paper plate or piece of cardboard to spread it out and keep it cool, then transfer it little by little to the part. As long as you get it out of the cup quickly and spread out to a thickness of about 1/2" or less you'll have more time to work with it.