As far as I know, the Odoo follow the modularity and that is the reason behind become very very strong into the current market
My personal advice on this to understand the client's need in a very profound way then try to cover all the requirements with existing Odoo flow. Explain to them why what the currently has in a real case.
So that you should think about the real client business flow and cover all the scenarios with default flow of odoo and add very very fewer fields and its field structure details into your new module.
As per my perspective
1) Is this advisable?
- As per the client's need and requirement of existing client business flow.
2) what would happen when updates are given in the module? ,
- If the client gives you a new update or want some more additional functionality then you should create a new custom module based on the new business flow and add the dependency with base modules of Odoo.
3) Is it better to start a new module?
New business flow has more and more complex needs a custom module to solve the current problem of the client. The new module helps you to arrange the proper structure and maintain a very long time.
4) Is it possible to copy the existing modules to have them as a basis?
-Not good practice to copy the existing module but better to add the dependency of all other existing modules as per the client business flow
4a) is it recommended?
- Not recommended for the same
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