Trail Boss: I take exception with the statement that "Trail Boss is fast and forgiving". I've used a few #'s of TB, in pistol caliber cartridges for shooting Cowboy Action, and fast and forgiving aren't among the descriptors I'd use to describe this powder.
It is a bulky powder, and when loading with a progressive press it is easy to see a double charged case. IOW, even at low charge weights it will fill the case more than other powders, such as Bullseye. This is a good thing, making it a safer choice. It is a bit more expensive to use, a 1# container only has 9 oz of powder in it.
Another characteristic is that for the pressure it has, TB yields a fairly low velocity. Full power hunting loads have enough pressure to seal the chamber, but when the powder charge is reduced for target loads or Cowboy loads, one of the issues is that below a certain pressure, the brass case does a poor job of sealing the chamber, cases come out sooty, and the gun gets dirty. Good ammunition should seal the chamber. The pressure curve for TB is characterised with a high peak pressure of short duration. This short (relative to other powders) duration pressure curve isn't long enough to give high velocities. IOW, it has finished burning before it can really push the bullet out of the barrel. In comparison, Black Powder has a low pressure peak and is still burning as the bullet leaves the barrel.
One of the characteristics of TB that makes it unsuitable for normal shotgun loads is that it does not work well if compressed. It is fluffy, easily compressed, and that changes it's burn rate. Pressures are high and unpredictable.
Your load has 8 gr of TB and the 2 balls weigh 185 grains, and if you add the wads, your total projectile weight is probably over 200 grains. My WAG at the pressure of your load would be 25,000 psi.
That would be more pressure than I'd be comfortable with.
Your reloading recipe of using a nitro, fiber, 2 balls (small enough to go through a choke), card wad and roll crimp is sound reloading practice, IMO. However I feel your choice of powder could be better.
Without doing much research, I lean towards Unique, that would be the fastest powder I'd use. As far as something I'd recommend, perhaps 2400, or even Black Powder.
https://smallboreshotgun.com/reloading-410-slugs/40-caliber-410-pumpkin-ball-round/
https://forum.castbulletassoc.org/thread/several-410-loading-questions/
Those were two sites with some info, your are pointed in the right direction, perhaps a bit more research required.