Hi Tomoji,
thank you for sharing your useful test info. BTW your entire work is really amazing!
I came up against similar limitation using Navio+ with M8N as stated here: RTK and Navio - #37 by generaldin
Other people here: RTKLIB-Anwendungen mit u-blox M8 - Seite 2 - Geospector GPS-Forum (vormals kowoma GPS-Forum) says M8N is capable to handle raw messages at 5Hz (even 10Hz) disabling glonass and SBAS.
Have you experienced this behavior?
(I almost agree with @jstromsoe. While Emlid is a company and it has to make money and I have nothing against this I think there was a communication error with the name “Navio+”, maybe putting extensively that M8N has not the same performances of NEO-6T in terms of raw messages and RTK, IMHO)