Your PCBS

Yes, 0.1 g*g/ Hz is indeed a high level but we need to know why people set up that high PSD. Is it in some standard like MIL etc.?

Do you know where the PCBs are actually used?

Do you have equipment you could take to the site and monitor g-levels your PCB faces? There are tiny pickups these days which can be mounted on PCBs.

Have you been aware of HALT and HASS techniques used for reliability improvement of Electronic stuff?

Regards

Deepak