Dynamic Mandatory Fields

You’re using folder defaults correctly for Service Type and Retention Label, but the remaining metadata must be enforced according to Service Type. SharePoint’s Required setting is static; library validation can block a save conditionally, but it will not add or remove the Required indicator. First, make Service Type, Retention Label, Document Type, and Status required, because every rule needs them. Set Fiscal Year, Building, and Vendor defaults to None. Then open Library settings > Validation settings and add:

=AND(IF(OR([Service Type]="Admin",[Service Type]="Events",[Service Type]="Janitorial and Maintenance",[Service Type]="Parking and Transportation",[Service Type]="Supplies and Equipment"),NOT(ISBLANK([Fiscal Year])),TRUE),IF(OR([Service Type]="Events",[Service Type]="Furniture and Moves"),NOT(ISBLANK([Building])),TRUE),IF(OR([Service Type]="Events",[Service Type]="Waste and Recycling"),NOT(ISBLANK([Vendor])),TRUE))

Add a message naming the conditional fields, then test one upload in every folder. Keep referenced Choice columns single-select. If users need visibly different required fields, create service-specific content types and set each column Required or Optional there. Users would need to choose the matching content type.