Hi Tim! Can you tell me how did you created this server action to manage to turn a sales order "fully invoiced"?
Thank you!

I am not the best at this but I will try to lay out the steps. I suggest trying it first in a stagging environment or on runbot.
Turn developer mode on or install the app : https://youtu.be/wGWHAX68Rt8?si=-fQPDFS9uM0gu_zU
From the home screen simple start typing 'server action' and the option will appear.
Click new.
Model is sales order
Action is update record
Field is invoiced status
Evaluation type is value
Value is invoiced
Save
Click create contextual action
Goto sales application and refresh
Action should appear in the action drop down

If you are new to server actions I suggest watching some of this Odooer's videos. Some great tips:
https://www.youtube.com/@odoo-it-yourself/search?query=server

Likely in the view_order_form, notebook. Currently identified as invisible.

<group name="sale_info" string="Invoicing">
<field name="fiscal_position_id" options="{'no_create': True}"/>
<field name="analytic_account_id" context="{'default_partner_id':partner_invoice_id, 'default_name':name}" attrs="{'readonly': [('invoice_count','!=',0),('state','=','sale')]}" groups="analytic.group_analytic_accounting" force_save="1"/>
<!-- <field name="invoice_status" states="sale,done" groups="base.group_no_one"/> -->
<field name="invoice_status" states="sale,done" readonly="0"/>
</group>