Don't worry about people complaining that it is a short game. Most people don't understand that it takes time. Games in similar genres are a bit more established such as Tales of Androgyny, Futadomworld, Corruption of Champions, Trials in Tainted Space, Warlock and Boobs. These have been in development for a while.
One way of making the game a bit longer without creating more scenes is making it more event based. All of your scenes are integral parts of the story. If you made the game as an RPG maker game where a character could move around, and at certain points, could trigger events, that would mean you would have to make less scenes for each gameplay event.
It would take time for the character to those scenes so there would be greater gameplay time. Creating events that would trigger would make the game feel more interactive and less like reading a story/comic. This would increase the gameplay time while requiring you to make less drawing and background to trigger scenes.
Also having loops in the game, such as a way to make money, loyalty, etc, where the character would have to grind in order to get to the next point is another way to increase the gameplay time that other games have implemented.
RPG maker for creating character movement is rather easy and so is making loops in ren'py to make the player feel like they have more control over the events and extends the playtime. Something that could be considered.