Unfortunately, you cannot address this with a trigger or other automation, because the system's validation for required fields takes place prior to the execution of
before triggers. From Triggers and Order of Execution:
- Loads the new record field values from the request and overwrites the old values.
If the request came from a standard UI edit page, Salesforce runs system validation to check the record for:
- Compliance with layout-specific rules
- Required values at the layout level and field-definition level
- Valid field formats
- Maximum field length
(Emphasis mine), followed by
- Executes all
Your solutions will all be at the user-interface level, and one option would definitely be a Lightning component to support Lead creation.
You don't necessarily need to go all that way, though. This objective sounds to me like it could be achievable using a Quick Action that prefills some placeholder value for required fields you don't want the user to have to enter, which would prevent you from having to write any code if you're comfortable with guiding users into that creation flow.