I have an existing protection group snapshot schedule

Gotcha, so you can’t use the Powershell SDK to change core functionality, but you can write a script to take a snapshot at will and apply retentions. You could then disable the builtin automatic snapping and use task schedule or an orchestrator platform to take the snap at the specific day/time you want. Just be sure to build in alerting (email or something similar) so you can have confidence the job is running