Not a problem, now more and more routers can handle what is needed to work with Orange in France.
Nice to see you here
OLT (SFU - so L2 version) - does exactly what OLT want's it to do.
This is usually sent as ExtendedVlanTaggingOperationsCfg MIB.
You can see it with command:
omcicli mib get 171
So ONT will behave exactly same way, as OLT wants.
Remember, this is generally telco-controlled device, as a part of telco network. All private persons putting own ONTs are a bit intruders
In theory, it is possible to enforce untagging of particular vlan (and tagging in uplink direction) by violating OLT rules, but - this requires lot of precious time to implement and test.
And other issue is - that as a result you will loose pbit information (that is a part of VLAN tag).
And it might be difficult to do separate pbit with DHCP reqests and separate for normal traffic. That would require involvment of DSCP and - no way... even as for hobby.
It will be simpler if you use small, managed switch to remove tags.