I have a few comments/questions:
- Your first image looks like you have set your locale to English in Settings > Language and Dictionaries > Select your Language. Is this true? The reason I ask is that if you set locale to one of the Chinese options then I believe the Kobo GUI (e.g. the main Book list) uses one of its built-in CJK fonts rather than Georgia/Avenir.
- Regarding the NotoSerifSC font files. It's not enough for the external filenames to be "disguised" to look like Georgia and Avenir Next, the *internal* font names also need to be "disguised". So, yes, you will need a font editor to do that.
- I notice that the NotoSerifSC files are all .otf, not .ttf like the built-in fonts you're replacing. Is a simple filename change enough to "convert" an otf to a ttf?
- The 8 built-in ttfs you're replacing are roughly 400K in total. Your proposed 8 replacements are almost 90mb. That's quite a big difference.
For items 3 & 4 I don't know for certain that those are problematic, but if I was trying what you're planning I think I'd be doing my experiments on an expendable old Kobo rather than my shiniest newest device
