Can you run benchmarks like 3DMark Nomad without issues?

Try disabling background task like AntiVirus (Webroot), Corasir, Elgato. Disable Steam Overlay. Try minimal boot with msconfig. Maybe something interfere with the nvidia drivers while access them.
Maybe FFXIV also has a problem with multiple GPUs. Try disabling the Intel iGPU in UEFI/BIOS for testing purposes.

The Problem have to be on your PC, i played without a single crash (dxerror oder this one) since DT early access, sometimes FFXIV just shows issues, like unstable RAM or GPU. Or its just broken nvidia drivers, since its using DLSS now.