Welcome to 11 more days of telenovela in manga form!
As many readers correctly guessed, Rio's marriage is absolute dog water, contrary to her insistence on telling her friends everything was normal. It's so depressing that, looking back, a huge reason for the downfall of the friend group is that no one could be transparent. I really want to believe there was a timeline where the girls all helped each other out before everything came crashing down.
Rio is a clear case of a trophy wife. I mean, the way her husband forces her to cook dinner for his friends at home, when he is wealthy enough to eat anywhere he wants, and Rio's only role in said dinner seems to be just a waitress that he can display to his friends, was pretty telling (seriously why call her to the table just to pick up the plates???). He seems to be rich enough to hire an actual maid and chef to his the house for crying out loud.
Rei was a shitty husband, but his shittiness stemmed mainly from laziness and selfishness. He wanted no responsibility but all the benefits.
Itsuki, in certain aspects, is worse than Rei. He is intentionally malicious (look at his language when talking to Rio) and seems to get a kick out of keeping his wife on a leash. He could have everything in the world with his wealth, but he seems to much more enjoy playing with Rio's mind. Perhaps being a sociopath is his preferred sport.
It's kinda insane that, despite Mahoro having the worst time of her life with Rei, you can still look at Rio and think maybe she didn't have it much better, if it all.
I am so glad that the author gave us a proper, in-depth look at Rio's life. It completely recontextualizes the main story and makes it just that much more rich and engaging. Not to absolve Rio of guilt, but to just show that this was a horrible situation all around.