Problems flashing back to TW 6.0

I have the G900P from sprint. I'm currently on a cm 13 rom, but I want to update to the latest stock touch wiz rom. There's no flashable .tar (for odin) or .zip (for twrp) on XDA I can find, only an update zip that's around 900mb. So I have to flash an old 5.0 stock rom, right? But the problem is I can't accomplish that. Any Touchwiz ROM, either custom or stock, I can't successfully flash anymore. I've only been able to run a aosp rom for the last couple months, and I think it's a partition problem. When I do flash a TW rom, it says success in TWRP (after wiping internal sd, data, caches, and system) but when I boot it'll bootloop or say optimizing apps, then bootloop. I've read around, and I think I have to flash a new .pit file. I'm just not sure how to do this/where to get this file. I have a spare s5 on 5.0. Should I try updating that, make a backup, and transfer that to my current s5? Thanks