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What operating system and version of calibre are you running? Using 5.11 under Windows, Linux and MacOS Big Sur, I do not get an unsupported firmware error with the latest firmware installed on any of my Kobo readers.
This suggests that you are not running the latest version of calibre.
As for the custom fonts, if the CSS for an epub lists a font or type of font, i.e, font-family: Georgia, serif;, since the Georgia font is present, it will be used.
For font names, there have been many discussions over the years. The internal font name must match the font file name, the correct -Regular, -Italic, -BoldItalic or -Bold (-R, -I, -Z and -B also work) must be appended to the font-file name, etc.
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