I have not found BH209 to be corrosive. IMO, it is essentially smokeless powder with the bulk of BP. I bought a container when it first came out (at about $30). It works great in inline muzzeloaders but it is considerably more difficult to ignite compared to BP and other BP substitutes.I’ve heard of some using Blackhorn 209 as a Trail Boss replacement. I can’t comment as I haven’t used it, but from what I’ve read it is a very low pressure powder similar to Black Powder and only mildly corrosive. It is said that it can be cleaned within a week of firing with Hoppes #9 with no ill effects. It is rather expensive though.
Here is an inline muzzleloader comparison of BH 209 vs Alliant BlackMZ which I heard is rebadged American Pioneer Powder. The smoke from BlackMZ is a much closer emulation of real Black Powder and is corrosive if left uncleaned for a long time.
For reduced loads, I've found that there is nothing that Trail Boss can do that Clays, Red Dot or Titewad can't do other than fill up a case and prevent negligent multiple charges.