Given that the inch was redefined in metric terms you should just accept the win — or do you not use 6.35mm hex bits in your tools? And will you not be using any 1/4″ or 1/8″ tooling?
For routers, there aren’t many which don’t match the de facto standard of 3.5″ (or as noted, one can use 89mm) — and I’m not aware of any smaller units which will accept a 12mm shank — even the Bosch Colt which was the darling of the CNC world for a while wouldn’t, and even the custom milling motors such as the Mafell FM 1000 WS I just bought top out at 8mm (5/16″)
My understanding is if you don’t get a larger router of a diameter as noted above (the Metabo HPT formerly known as the Hitachi M12VC is popular) you’d need a spindle, either 65mm or 80mm diameter which has an ER-20 collet.
Note that you’ll probably also need to get a custom collet if the unit doesn’t come with a 12mm collet, so check the usual suspects, Elaire Corp. and the Precise Bits folks.