I agree it'd have to be another mythical, someone who has also lived a long time and experienced similar trauma. Trauma possibly as the result of persecution or possibly war? I feel like it should be mentioned in the main story, but would be up to MC to help get things set up (R&D). I think that the content could go over some of the same things that were covered in the ruthless route in terms of Asterion's past experiences with other masters.
As for the length of the actual content... What if a small part of the therapy is part of the main story, and then it's up to you to decide if you want to hear more (R&D quest). Again with the implication that Asterion is still going to therapy regardless of what you do. At that point you would unlock the option to do the R&D quest and you can decide to complete the quest to essentially learn more about Asterion's therapy.
I also think it should be heavily implied that at the end of the VN Asterion is still in therapy, it shouldn't be a one and done sort of thing that magically fixes everything. So whether or not you decide to do the extra R&D quest to learn even more about his therapy, it will still be shown that he has been doing therapy the whole time and will continue to do so in the future.
How to introduce a character who is a therapist? Well the hotel takes in people who are without a home, or who are lost. It would make sense that a lot of them are running from abuse or trauma. It wouldn't be too big a leap to have a guest come to the hotel who is a therapist who decides to stay (temporarily or as an employee) to help some of the other guests and employees work through their problems.
Anyways, I hope this gives your team some ideas, and that you'll consider adding an option to let Asterion have some much needed therapy in a future update! Loving the game so far, thanks!