2.5g is kinda a odd ball. Enterprise stuff has long used 10g ports and there is little reason to use 2.5g or 5g stuff becuase there is no longer a huge price difference.
2.5g seems to be something they are selling to home users even though I am not sure why they did not just use the 10g stuff.
In any case your problem is the only router cpu used on consumer grade routers that can do this is BCM4908. You will find this chipset in every router that I know of that has a 2.5g wan port. Maybe there is another chipset in development that has the ability to run more 2.5g ports but this information is likely hidden away in much more technical forums that this one.
There are companies like mikrotik that make more advance routers but...