Break out the concrete floor and run the new floor at the same level as the existing floor. It’s the best option long term.
This will avoid any changes in level between this room and the rest of the building.
You need to consider the knock on effect of raising the floor level.
You’ll need to replace all the skirtings and door facings.
The door apertures into the room will be reduced in size which might make getting furniture in and out of the room a royal PITA. Window cill heights might not meet the minimum required for building regs. Sockets and switches will be at the wrong levels.