Use a Powershell .ps1 file to launch Access. As an example, name it Launch.ps1.
Start-Process -wait "C:\Program Files\Microsoft Office\root\Office16\MSACCESS.EXE"
"C:\Program Files\FreeFileSync\FreeFileSync.exe" C:\Users\xxx\Documents\Backup.ffs_batch
Then in the bat file, have it launch Powershell with a hidden window pointing to the launch script.
start powershell.exe -WindowStyle Hidden -file c:\scripts\launch.ps1
When you run the bat file, the command prompt window will briefly appear, launch Powershell and then go away.
The PS window will be hidden and launch MSAccess and wait for it to terminate. Then it will run your sync tool.