Note: SMB / SharePoint is surely better with a proper server/computer. A Raspberry Pi running OpenMediaVault (Debian) is a more common and robust option (still 5v low power).
If you are actually in an "it must done in OpenWRT" scenario, Click Here for my Samba config file: OpenWRT_Samba-config and see below. Also, please use a NTFS or EX4 format. 🙂
...While my sharing method wasn't actually adopted by others, I still think it is good to know!