Custom Field in Document Library

[Mahi_Hailu]

Hello,

We are currently moving from SharePoint 2010 to online but blocked by not being able to implement a custom 'Document Number' field in our document libraries. This field should be composed of the Library Name it's in,  Index (Meta Data column) and some other existing fields. So it'll be [LibraryName]-[Index]-[...]. 

We tried to use JSON column formatting to accomplish this but the library name and  fields from a term store can not be pulled. Another alternative we tried is using Flow. The problem with that approach is that we need this field applied to around a thousand sites with around 20 libraries in them and there is no easy way to deploy a flow to every library in the entire sites. 

Has anyone tried doing similar customization before? We are all new to SharePoint development so any suggestion that can guide us to the right direction is appreciated. Thank you for your time :)

Thanks,

Mahi

[#2]

What about the programmatic approach by means of Webhooks or Azure Functions running on a schedule?

[#3]

Thanks Juan, we want this field needs to be processed on every document upload and preferably with very minimal delay. I was just looking at remote event receivers. If there is no simple way to do this, can we try that?