Just for future reference, that I2C shifter you are using a lot of times does not work well with addressable LEDs. A test of different ones can be found here: Levelshifter Analysis
As for them still rebooting when the LEDs are off (black), I would still try disconnecting the LEDs from the controller/power supply just to see if the board still reboots.
Do you have these setup with Home Assistant? If you do HA has been known to poll the Esp too frequently causing lock-ups and reboot issues.