Everything is grounded already. Please dont connect some random stuff with each other.

The PSUs are grounded through the power cable from the wall and all the hardware is grounded through the power cables coming from the PSU. Connecting a mounting hole to the PSU case could only trigger "Case Open" detection some mainboards have and thats useless....or in worst case you create a short somehow and break something with the addiditonal cables added.

Just watch out not to mix stuff up and power each piece completely from the same PSU (e.g. Riser + GPU from same PSU, CPU Power + ATX power from same PSU etc.)