it depends on how your company and maybe also external will work with this files in the future. This is the most important point in my opinion.
Use much as possible folders or document sets. Nobody wants to enter metadata in their smartphones. Add later metadata columns for quick search results.
Keep also in mind there are also limits and boundaries in your online tenant you can not change like on premise. 5000 max items "per view" (without index) and 12 lookup columns "per view" for example. In SharePoint Online, file names can have up to 256 characters. Folder names can have up to 250 characters ...
In my opinion it's not important how to set your file name specifications. At the end you will search the entire file anyway. But for sorting I recommend to use "YYYYMMDD" at the beginning of a filename.
Basically: Keep it simple and integrate a pilot group! ;o)
Following are some points you also should pay attention.
1. SharePoint structure concept (internal / external). -> Max. three clicks to target. Dissolve your deep file structure in Team Sites or individual libraries. 2. Where to use communication sites (Hub Sites) and associated Team Sites? 3. Search for invalid file names and file types for your OneDrive, OneDrive for Business, and SharePoint 4. Make a test migration with Sharegate or AvePoint 5. Where to use folders, metadata and / or document sets 6. Integrate the Teams Service 7. Permission concept (Sercurity and Compliance settings) 8. Are there any processes where are additional O365 Services can be helpful (Teams, Flows, PowerApps, etc.)? 9. etc.