I would personally not call it a good mystery NTR.
- The son's POV is unnecessarily restricted in both games, the SON has access to the the other kids account but during his route you cant scroll the text messages for no reason other than the developer chose to show those things "later" when you are on the Bull POV where you again switch to the SON POV and can finally scroll down.
- The voyeur sections of the SON are pretty much shown as text and black screen and only one scene behind a curtain and one full peeking at sex. the others are kind of hearing stuff. or being around unaware which is mentioned in what amounts 2 to 5 lines. as a side note I think the son has 0 motivation to peek on her mother, besides again the developer designating him as the game's cuck. the character feels apathy towards the relationship the mother and the bull have and is more just curious about seeing his mom naked and having sex.
- the whole voyeur content is optional, you can choose to have the SON miss all the chances to see action, in fact you have to do so in order to unlock some of the scenes. (which are just variations)
I think what makes this series good is the shota bully/nagging sex. the artwork which I think is peak, the narrative ( when you follow 1 of the stories or you are not desensitized by skipping) and the sex scenes. I literally only replayed this game because on the CGs that are posted online there i saw a scene with red underwear and i wanted to unlock it.
the initial exploration of the diary (the hidden NTR part)
Like I say Playing it with a guide would be the best recommendation as for the mystery NTR i think it is not that good. but enjoyable. that is to say it has mystery but if you play it for the mystery you will be disappointed.
maybe I should make a guide compiling all the other half broken guides that helped me finish the game.