3.3V as the pull up source is acceptable even if DVDD is 1.8V. But NVCC_I2C of the I²C domain in SoC should be 3.3V (the same voltage as the pull up source for PMIC) and DVDD in PMIC always needs to be 1.8V.
The output voltage for BUCK8 can be adjusted by U-Boot in the initial startup depending on the memory type in use. The voltage change handled by U-Boot is acceptable for PMIC because the validity in the power sequence is confirmed by NXP and no other sideband effects are found during their validation.
BUCK6 is initially ON as the default setting and its output voltage is monitored during the power sequence as an one of the triggers to de-assert POR_B, the power good signal to SoC. So it is not recommended to turn off BUCK6.