At what point the inner width is too much? 23mm is what my gravel bike gets, I'd be afraid of damaging the sidewalls on a 25c tire. Maybe there's finally a reason for tubeless road.
You're in experimental, but probably not yet dangerous, territory when the inner rim width exceeds about 75% of the true (not nominal) tyre width. You can get away with more at the back than at the front, because the front leans more.
At what point the inner width is too much? 23mm is what my gravel bike gets, I'd be afraid of damaging the sidewalls on a 25c tire. Maybe there's finally a reason for tubeless road.