Design advice - Custom Document Id provider (sort of/maybe?)

Dear Community,

We are writing an app add-in for Sharepoint Online.  For every document that's uploaded via our app,  we will assign it an ID that we've generated.  The value can be empty/null too.  The problem is how and where to store our ID in Sharepoint.  The solution needs to cover the following:

• When a document is moved to another document library the ID column value moves with it.
• It works across all file types
• The column is indexed and searchable.  Need fast retrieval
• It needs to work for users with basic subscriptions, not just enterprise.
• That it doesn't interfere with sites that already use the out of the box Document ID service
• The value shouldn't be easy for end users to view/edit.  It's for our internal bookkeeping only.

Is this possible to do?  An alternative choice is to create a "list instance" in the application add-in.  That list will contain our "documentId" and a couple of other required columns. If that's a better choice do you know how large add-in list instances can get before there's trouble?  Is it 30M rows?

Appreciate the expertise and guidance,

Sharon