Edit: to add detail to original question

you can override the "copy" method of the project.project model and modify its behavior. Here's an example:
from odoo import models, fields, api

class Project(models.Model):
_inherit = 'project.project'

def copy(self, default=None):
if default is None:
default = {}
default.update({
'name': self.name,
# Add any other fields you want to copy from the template
})
return super().copy(default)

In the code above, we override the "copy" method of the project.project model and provide our custom implementation. We update the "default" dictionary with the desired field values from the template project. This will ensure that when the copy operation is performed, the new project will have the desired field values.

Make sure to restart the Odoo server after adding this code. Once implemented, you can remove the super(Project, template_id).copy(default) line from your "action_create_project_from_template" method.

Note: This approach assumes that the "signage_project_template_id" field on your model contains the ID of the template project you want to duplicate. Adjust the code accordingly if your implementation differs.