treequell Synth GrapheneOS Keyboard depends on AOSP Keyboard for language support. I am not sure about Japanese Romaji specifically, but AOSP has stopped maintaing and dropped support for many languages. Unfortunately, the only good option is using a third-party keyboard app.
boarim I have the same issue here. I ended up with the Google app GBoard as it has Japanese input support, although it’s bloated and a proprietary software. I would love to see GOS use a customized AOSP keyboard with CJK language support.