You might wanna extract separate partition images from the ffu then edit the MainOS image but you can't repack them into a new ffu. You can only repack those images into a flashable zip file. If you're not in hurry, I can write a full tutorial next week if you need one. Otherwise, you can experiment yourself with Interop Tools, OSFMount and Total Commander. Just be very, very careful cause you might turn your device into a brick
I second dxdy.
Ya Interop tools and sftp or Full MTP access is what I use.
Go to C:\PROGRAMS\COMMONFILES\XAPS
Remove what you do not want. But be careful not to remove stuff you may need later, like wallet or other foundation apps. Also if you Hard reset they will be gone next time around. Secondly I have found some OEMs use another folder I think under C:\PROGRAMS\COMMONFILES\OEM\PreinstalledAPPS or something C:\PROGRAMS\COMMONFILES\OEM\Preinstalled......
The main folder is the XAPS folder though, that gets used during OOBE provisioning.