I actually came up with my own solution and it works quite well for some of my HVAC customers that have D365 techs offline 90% of the time in areas without internet service.
I use a power automate flow to create all the Sharepoint folders and opportunity document location records at the point the opportunity is created, so the process isn't driven by the first user who click on the documents grid on the documents tab.
The Sharepoint folder creation power automate flow uses a specific naming convention that matches the opportunity name so it's obvious to the end users what document folders go with which opportunity.
Then, the users map the Sharepoint document library to their OneDrive, choose the option to keep a copy of everything locally on the device, and then give time for a full sync to happen.
At that point, BOOM! - Full Offline Documents, albeit NOT via D365 mobile offline app, but it gets the job done, and the techs can upload their estimates, photos, etc to the one drive folder, so it syncs up once they get back into an area where their phones pick up data service again.
