Hm. Their current Japan esim page states network is KDDI 4G https://roamless.com/esim/japan-esim I haven't spotted many MVNOs on KDDI. Navitime (map data provider) has a KDDI 5G travel esim offering https://japantravel.navitime.com/en/esim/ Rakuten Mobile roams on AU as a partner network in some areas https://kddi-hikari.com/column/rakutenmobile-5g-area/

Had just replied on the Osaka World Expo thread with the Softbank datapoint but good to know another carrier option is available.

Came across this in case someone is looking for bands, base stations, and a speed roundup of low cost 5G domestic offerings https://kakuyasu-sim.jp/band#5G-SIM