Trail Boss would be the obvious sub choice, but as you probably already know, is 'unobtainium' right now.
Howdy
Trailboss is not a Black Powder Substitute. This is a common misconception.
Trailboss, as the name implies, was developed for the Cowboy Action Shooting crowd. Because a lot of the public believes all CAS shooters shoot Black Powder, a great many think Trailboss is a BP Sub. Many (but certainly not all) CAS competitors load down their 45 Colt cartridges so that they recoil like a mild 38 Special. This is so they can fire their pistols very, very rapidly without recoil upsetting their aim. When you put 1/2 grain of Whiz Bang into the cavernous 45 Colt case it usually does not work very well. Inconsistent ignition is common. So Hodgdon came up with Trailboss. The fluffy, donut shaped grains take up more space than most other powders. Here is a photo showing Trailboss grains on the left and Unique grains on the right. The idea was using Trailboss one could load fairly light loads into large capacity cases and cut down on the amount of empty air space in the cartridge. This is in turn would cut down on inconsistent ignition .
Regarding getting consistent velocity with fairly light loads, you are probably correct that Trailboss is a good choice because it eats up so much of the empty air space in the huge 45 Colt cartridge.
One thing you might try is using 45 Schofield brass instead of 45 Colt brass. Less powder capacity than 45 Colt, so light charges can be more consistent than similar charges in 45 Colt. You don't mention what type of conversion cylinder you are using.
I have the R&D six shot 45 Colt conversion cylinders for my Remmies. I usually shoot 45 Schofield cases in them with a 200 grain bullet to cut down on powder capacity and to be less punishing to the Remington frame.
When originally made, the counterbores in the R&D cylinder could not accommodate the larger diameter rims of 45 Schofield (.520 in diameter vs .512 for 45 Colt.) I had a gunsmith open up the counerbores so the Schofield rounds would seat. I'm pretty sure this cylinder now comes with counterbores big enough for both 45 Colt and 45 Schofield these days.
I only load 45 Schofield with Black Powder, but there is data for loading it with Smokeless on the powder company websites.
If you go to the Alliant reloader's guide there is data for Schofield with American Select, Bullseye, and Unique using a 230 grain bullet.
https://www.alliantpowder.com/default.aspx
Hodgdon has more reloading data for 45 Schofield, with loads for 200, 230, and 250 grains with 231, Clays, HP-38, Titegroup, Trailboss, and Universal.
https://www.hodgdonreloading.com/
I buy my Schofield brass in bulk direct from Starline, but you can buy a package of 100 cases from Midway USA.
This is a photo of all the cartridges I load with Black Powder. Left to right they are 44 Russian, 45 Schofield, 38-40, 44-40, 45 Colt and 45-70. This should give you a good idea of how much more case capacity there is in the 45 Colt case than in the 45 Schofield case, which makes the Schofield case a good candidate for efficient loads with light loads of Smokeless powder.
For what it's worth, my standard Smokeless load for 45 Colt is 7.5 grains of Unique under a hard cast Smokeless 250 grain bullet. My reloading notebook. says that velocity out of a 7 1/2" 'original model' Vaquero was 781fps to 828 fps, with an average of 801fps.
Also, you might consider using a lighter bullet than 250 grains. You don't mention it, but I suspect you might be looking for slightly reduced recoil. If so, reducing your bullet weight from 250 grains to 200 will have a very significant effect on reducing recoil. Even going to a 230 grain bullet will significantly reduce recoil. You must, of course, adjust the powder charge for the lighter bullet. Believe it or not, generally if you use a lighter bullet, the powder charge usually goes up a little bit from what you were using for a heavier bullet.
What ever you do, DO NOT USE A FILLER WITH ANY TYPE OF SMOKELESS POWDER.
