Woah... Maybe I just haven't seen around enough, but this is the first time I've seen furigana used to this extent in hentai manga. Furigana at this amount in media is usually used for people learning Japanese (typically youth or non-native learners). Since this type of content is obviously not for minors, this feels like an effort to make this more accessible to foreigners. The individual releases didn't even have it e.g. ( https://e-hentai.org/s/5c709f17b9/2654524-4 vs https://e-hentai.org/s/b68388d54c/3954884-105 ). A welcome consideration. Will definitely be supporting Guglielmo even more now.
Actually, tankoubon volumes from WANIMAGAZINE usually add extra furigana for kanji. To give a few random examples: ① This one & That one ② This one & That one
As a side note, it's incredibly rare for a commercial tankoubon to compile self-published doujinshi like this one does. And to be honest, the typesetting in self-published doujinshi can often be a bit sloppy (lol). For example: This one: Inconsistent font sizes on this page. That one: The layout on this page was visibly cleaned up.
It just goes to show that publishing a tankoubon is never a simple "copy-and-paste" job—they put a lot of work into careful editing. :-)