Landlord/home owner has a legal obligation to report a foreigner staying, within 24 hours. But if they don't, better it's done by you than you get into trouble for it. The Visa holder has to report where they're living every 90 days
My Thai lady and I were split up going through immigration going out and coming back to Don Mueang. Awkward, as it was her first time at an airport going anywhere and it did cause some anxiety
I just Googled 'condos to rent bangkok' and found several agencies with many places you can choose from. Even some of the tall buildings have apartments on the 4th floor, while others are offices or retail at that level. Your real problem is I understand Thai law does not allow private rentals less that 30 days. You could look on booking.dot com (for example) since they have many AirBnB type properties these days
So can come and go as she pleases right now, for max 3 months each time. The last time she can enter is 12 April 2025 and, if she enters on that date, can stay until 11 July 2025
Elements of Thai culture are indeed feudal and have been surpassed in many western states (although UK still feels feudal in many ways). That's exactly what it is 🤷🏻♂️