I load/shoot a lot of 357 Magnum. Been doing so for decades.
It would be helpful if you told us the purpose of the ammo you intend to load.
Since you have a 125gn plated bullet, I'm going to
assume you are looking for basic range shooter ammo.
Plated bullets are not nearly as sturdy as jacketed bullets and are not a substitute for jacketed bullets. They are an alternative for straight lead bullets. In the case of 357 Magnum, a plated bullet can easily be over-driven, leaving the inside of your barrel with a nice bright shiny orange (copper) coating. I know. I've done it.
With plated you don't want to exceed their recommended velocity limit
Most plated bullets' manufacturers print a velocity limit on their box. It's a bit misleading. Really, the issue is pressure - not so much velocity.
I don't load 125 plated bullets for 357, but I do load a 158 plated SWC. My current recipe is 7.0gn Unique. It yields 994 f/s 3"bbl; 1058 f/s 4"bbl; 1087 f/s 8"bbl; & 1251 f/s 16"bbl (carbine rifle). I believe this loading is at the edge of a plated bullet's capability - regardless of the printed velocity limit on the box.
Although I'm not a fan of Unique, it's nice and fluffy, so the fill level is good. And it's not so spunky-fast, so the pressure curve is gentle enough for plated slugs. Both of these characteristics are good for what you're doing and manifest even more so when moving to your lighter 125's.
I'm out of Unique (intentionally), so subsequent loadings will likely be with AA#5 - I still need to do the workup. AA#7 and AA#9 were mentioned. AA#7 would be a good choice; as the pressure curve will be soft. AA#9 might be too slow - depends on how much unspent powder you want to deal with while shooting/cleaning. Loading up AA#9 to the point where it runs clean, will likely bring pressures/velocities up to the point where plating breakdown becomes an issue.
Point is, with plated/357, you'd want to stay away from fast powders like Bullseye, W231, TiteGroup, etc. They will develop a sharp pressure curve that doesn't bode well for plated bullets. Move to an intermediate burn rate propellant like Unique, AA#5, AA#7, BE-86, Power Pistol (turned down a bit), etc. -- propellants in that range. Using slower powders would be ill-advised because it would be hard to get them to run consistently before pressure AND velocity becomes an issue. They should be reserved for jacketed bullets (IMO).