357mag using Titegroup

Xtreme Bullets has a reloading guide you can download on their website. They show a few loads for 125gr 357’s using 7.3 gr Tightgroup. You could back off to 6.8gr & go from there.
https://www.xtremebullets.com/category-s/50195.htm
Berrys Bullets site says you can use cast or jacketed load data for their plated bullets that I believe are pretty much the same as Extreme bullets.

Both Berry and Xtreme both have maximum velocities recommended for their bullets. You will lose accuracy and key hole your targets if you exceed their recommendations. Xtreme also recommends only a light taper crimp.

7.5 grains might get you 1300 fps with a 6 inch barrel.
With a 3 inch barrel, it will be just over 1000 fps.

Be aware that the cast loads are held down not because of pressure limits, but because of leading. When everything is done properly - hardness, lube, fit - cast bullets can be driven as hard as jacketed without trouble. The load manuals tend to hold down pressure because they know a lot of people aren't going to do everything properly and don't want to hear complaints about leading.

Short version: plated bullets can generally be used with any published data, as long as they aren't driven faster than the manufacturer warrants.

The Xtreme Bullets data shows the pressure they measured for the published charge masses, which they tend to keep around 30,000 psi for 357. SAAMI maximum is 35,000 and CIP maximum is 300.00 MPa or 43,511 psi. You mentioned "mid-range" loads. I don't know if by that you mean in the middle of the range of pressures or in the middle of the range of 357 velocities. I would suggest that at around 30,000 psi, Titegroup will produce velocities that are on the low-end of midrange for a given barrel length. It is a "fast" powder. With it, pressure rises fast to the peak and then falls off fast, producing less velocity than more progressive, slower-burning powders. It's suitable for the lightest loads. It works great for mid-range loads if you load closer to the maximum pressure.

What I'm saying is that if you load in the middle of published data for Titegroup, you're going to get light loads. You're going to have to work-up to published maximums to get mid-range loads. It can be worked up to CIP maximum pressure to get velocities that might otherwise call for a powder like Power Pistol or 2400. These would be top-end loads for Titegroup, but mid-range for 357 Magnum velocities for a given barrel length.

I've loaded a lot of 125gr Extreme bullets with Titegroup for 357. I ended up in between the lead data and jacketed data with a load of 5.9 grains. They are accurate with mild recoil

Leading is a failure of fit or lubrication. With bullets you buy that's not adjustable in a meaningful way. If your just off a known accuracy load you can try and add some tumble lube. When using tightgroup I stay in the mid loads and below. It's very accurate and good in that area. I don't use it for magnums.

With a plated bullet, if there’s no published data for it, and really fast powders like TG, I start at the higher end of cast data or the lower end of jacketed.
Extreme also has this:
“
Loading Facts: Due to the large case capacity, it is not recommended to use 357 Mag cases to load for target shooting. Instead, use 38 Special cases and loading data.”

I’d be interested in any range report.