Here is the first draft. Please add comments.
Mandatory - all fields: Add text at the top of the form above the first field saying "All fields are mandatory." Do not add quotations marks. Sentence case with a period at the end. Do not mark each field.
Optional - all fields: Add text at the top of the form above the first field saying “All fields are optional.”
Both Mandatory and Optional:
50% each: Add (Mandatory) in brackets next to label, above each Mandatory field. Title case. Do not mark Optional fields.
Optional fields are less than 50%: Add (Optional). Do not mark Mandatory field.
Mandatory fields are less than 50%: Add (Mandatory). Do not mark optional fields.
Font colour: Grey #737373 for all
Existing examples:
https://www.toronto.ca/business-economy/doing-business-with-the-city/claim-for-lien/claim-for-lien-submission/ (click next button to see the form)