Before anything else, did you do a proper decontamination of the logic board (remove shields, ultrasonic bath, Branson EC, IPA rinse etc)? If you didn't do this initially, then you should do it now.
Once you've done that I would start by going "bare bones". Disconnect everything from the logic board except the Battery and Dock. Connect an iTunes enabled computer and run 3uTools (www.3u.com) to see if the phone is still rebooting. If it is, then it is most likely a logic board issue (it could be the dock though).
If you suspect a logic board issue, check PP_BATT_VCC, PP_VCC_MAIN (which you already did) and PP5V0_USB. I would start by checking to see if those rails are shorted to ground. If one of these rails is shorted to ground, then you will need to identify what is causing the short. It could be a bad decoupling capacitor, conductive debris or defective IC that is directly supplied by those rails.
Then you move onto the PMIC and check the voltage rails it generates. The PMIC generates ~15 voltage rails. They are all important (for obvious reasons) but the ones to check first are as follows:
- PP_CPU & PP_GPU, PP1V8_SDRAM & PP1V2_SDRAM, PP_VAR_SOC & PP0V95_FIXED_SOC, PP3V0_Tristar & PP3V0_NAND, PP1V8_ALWAYS, PP1V0.
- The PMIC also generates, what I would consider secondary, yet still important voltage rails for the following sub-systems: PP3V0_MESA, PP1V8_VA_L19_L67, PP3V0_PROX_ALS, PP3V0_PROX_IRLED, * PP3V0_IMU, PP3V3_USB, PP3V3_ACC