Once you have disconnected everything, reconnect the bare minimum required to boot up the phone (and know it's booting). In most cases this will mean a display, a battery and either the power button cable or the charge port assembly to trigger boot.

For sustained use, all iPhones beyond the X need certain sensor containing parts connected to prevent a reboot after 3 minutes. Make sure you have a known good battery, charge port assembly and power button cable to prevent this.

Refer to the SMC Panic Assertion failed page for a more complete list per model.

With only the fewest number of necessary parts connected, try to recreate the issue. If your issue has resolved, start reconnecting parts one by one until the issue reappears (be sure to disconnect the battery in between).