I would say 'Tales of the Unending Void' is one of the exceptions with a well written MC.
Agreed the MC of that game was well written. There are many games where the story and characters are great but only a few where the MC is well written and compelling. I think Once in a lifetime, Eternum, Life changing choices, Trouble in paradise, Desert stalker & Maids and maidens has the best written MC's. Ravager also deserves a spot for try something unique and different.
BTW, for me Trouble in paradise is an underrated jem.
While this isn't directly related to fleshed out AVN MCs,I think Harem Hotel gets a pass for me.Even though the MC in that game is bland like many others, the narrative is self-aware about that and doesn't try make him be more important than he actually is. He's only the driving force for the real stars of the show, the LIs. All of them are fleshed out and are central to the worldbuilding and story of HH. For me,HH is how you should handle an AVN MC if you're going to make them a boring self-insert.
I definitely agree that Cari's games do have well written MC that feel like characters with their own personalities. And I get why most AVN make the MC bland,At the end of the day people play these for the porn and it's easier for self insert. But, if you're going to have the MC play an important role in the plot at the very least you could give them a personality that isn't cardboard.