How to customize invoices admin page

[NMod-IWL]

Hello,

I made a module that extends the billing part of WHMCS. To simplify, it connects WHMCS to external services to handle what in Italy is called "electronic invoicing" and customizes the native WHMCS behaviours of numbering, handling credit notes, zero-amount invoices, etc.

I have always worked in the backend side and added a little HTML part to the invoice detail page, but I was wondering the correct way to be able to edit the invoice list admin page. I would like to show in the invoice table the status of the invoices (e.g. pending sending, pending outcome, acquired, to be corrected, ...).
I did some research and the answers I found are:
1) modify the table DOM via JS with AJAX calls to retrieve the information about the invoices shown
2) create an entire custom page template.

Could it be that there is no way to tell WHMCS "add column named X and for each row of that column run the code Y"?
It would be nice to find more in-depth documentation than can be seen online...!

Thanks for any suggestion.

Nicola