Starline is HARD.
During the shortages when they were making only a little .357 I got some cases from them and it was like trying to resize diamonds. They were even worse than usual.
Nice thing about revolver cartridges that are based on old black powder cartridges is that COL isn't nearly as critical as in a small case like 9mm, 10mm, .40 cas
If the bullet has a crimp groove, all the better., just use that. Seating deeper doesn't help because a revolver cartridge is held by the roll crimp, unlike a straight wall semi that is held by neck tension with a taper crimp. I believe that XTPs have a groove.
With the light loads you propose, I'd suggest a light roll crimp. Moderate loads take a slightly heavier crimp. Using slow magnum powders like h110/win 296 people generally advise a heavy crimp to get a good burn started before the bullet leaves the case.
Are you sure you want to shoot targets with slow moving XTPs? If you are rolling in dough, go ahead, but for plinking and target I use much less expensive bullets than defense style.
Plated bullets are ok to about 1200 fps. Heavy plated plated bullets are almost like jacketed
For the best accuracy I use .38 HBSWC swaged bullets. Grease in groove lube for harder bullets is ok.
I mostly shoot polymer coated bullets, mostly Hi-Tek coated that cost about 8 cents each.