I've loaded and shot an honest 8-10,000 rounds of 454 casull. This is a different critter but also familiar if you dabble in magnum pistol cartridges.
Best you'll get is h110/296, I know Lil gun does similar with less pressure but Bob Baker warns against it and that's enough for me
Mid loads are great with 2400, it can be downloaded to suit needs.
Mild loads have several powders that work OK, I use clays with a 250 rnfp for just target shooting- a pound of powder lasts forever and it burns clean. Clays meters poorly for me so I load these with a dipper.
trailboss is out of the question,
No... trailboss is awesome in 454. It may be that it's out of the question as far as availability but as far as making sweetheart plinkers it works perfectly in any big ol case you care to put it in. It's literally listed as for use in ANY cartridge as long as load data is followed. I've used hundreds of these loads with a regular plain Jane 250 grain bullet and it saves me from the crud build up that's possible from using 45 colt cases. Trailboss is great.
In my opinion the 454 casull is all about versatility. That's not the common consensus but it's been my experience . Treat it like a 45 colt that isn't scared of insane loads. Of all the revolvers I've fired , the 454 chambered pistols have been the most viscous as far as recoil, more powerful cartridges are typically chambered in much heavier guns. You'll know when you hit you limit and I'd wager it'll be in the 50k psi range. The only thing you do by increasing velocity with this cartridge is flatten trajectory . A 360 grain wfn at 1500 fps is no more effective as the same bullet at 1000 unless you can't connect because the trajectory . It's going through whatever in front of it.
One last point to mention , this cartridge seems to have an extreme point of impact shift between bullet weights. It's not uncommon to be dead on at 10 yards with a 250 grain @ 1000 fps and then have a 300 grain at 1500 fps not even be on paper. It took time for me to recognize this for what it was. Each load seems to require individual sight in. I attempted and am still working on a plinking load that can be close enough to my full power load that they're both on paper, I'm close but far off too.
Good luck, anything that bears the casull name is a monster. Treat it with respect and it will impress you but to get it all right takes patience and effort. It's worth it.