[Renate]

On the Poke3, you can change the "serif" font (if you're rooted).
In /system/etc/fonts.xml:

    <family name="serif">
        <font weight="400" style="normal">NotoSerif-Regular.ttf</font>
        <font weight="700" style="normal">NotoSerif-Bold.ttf</font>
        <font weight="400" style="italic">NotoSerif-Italic.ttf</font>
        <font weight="700" style="italic">NotoSerif-BoldItalic.ttf</font>
    </family>

The least confusing way is to change the names here rather than renaming files.
Of course this will change the serif font for everything on your device.

Or you could use a different reading app.

Or you could hack Libby.