120 grains of ffg sounds like mighty powerful medicine, but when you're trying to reach out to 200 yards, than I guess that's the kind of powder load that you choose to work with.
One of the reasons for the less accurate first shot is that when your barrel is cold, pressure and velocity is increased. Believe it or not, a tiny bit of lubrication swabbed into the upper part of the barrel either before or after loading may help to reduce the added pressure and velocity and help to improve downrange accuracy.
There are 2 ways that you can try to do it, one way is to load your powder and then swab part way down the barrel with a tiny amount of lube (like Wonderlube), then load your sabot for the first shot. A small amount of lube residue might remain in the grooves to better help the sabot exit the barrel with better behavior.
The other way would be to load you powder and sabot down the clean barrel, then swab a tiny amount of lube 1/2 to 3/4's of the way down the barrel, but only a very light amount.
Some folks say that using a little lube with their sabots helps them to get more consistent groups, especially with that first shot out of a cold clean barrel.
If that doesn't work, then there are other things to try, but it's hard to say that you should tinker with what's otherwise such a good long range load.
There are TC Break-O-Way sabots which shed from the bullet more readily upon exiting the muzzle. Sometimes the brand and or design of the sabots may make a difference, even if it's just for the first shot. These TC sabots use a .44 magnum bullet (.427-.430 diameter) and can be found in bulk or in 10 packs with Hornady 240 grain XTP bullets.
There are other newer sabot and bullet styles too, but I was just trying to cite an example.
Buffalo Bullet makes .45 caliber 225 grain lead bullet/sabot combination.
But a lighter bullet will shed even more energy at the longer distances.
While you said that you want to push your bullets faster to help eliminate bullet drop at long range, I can't help but think that maybe if you went to a heavier bullet/sabot, you might obtain better stability out of your fast twist barrel, even if it's just for that first shot. You would sacrifice some range, but you might gain more first shot reliability. Some bullet weights to consider might be a 260 grain lead (Knight), or 275 to 300 grain bullet offerings from other companies.
Having enough bullet lethality left is always a concern when shooting at such long range. Sometimes heavier is better since the bullet will have more enertia to obtain penetration.
Also however, if you chose a pure lead bullet like a .45 TC Maxiball (255 grains) to use in a sabot, I don't see how you could get any better expansion at the longer ranges, especially vs. using a not pure lead bullet that's not as soft. So then the issue becomes bullet penetration vs. expansion, and at what range does your bullet choice begin to lose its effectiveness for either one purpose or the other.
Personally, when hunting I'm most concerned about that 1st shot out of a cold, clean barrel and how accurate it will be. I'm definitely less concerned about the accuracy of any 2nd or 3rd follow up shots, because if the first one doesn't shoot true enough, there may not be a chance for any follow up shots at all. So reducing the powder charge to obtain better first shot accuracy is not always a bad idea either IMO, if that's what's necessary to obtain the best 1st shot accuracy with whatever bullet is chosen.