Keep trying to hit topspin. A slice may be your eventual choice is such circumstances, but knowing how to hit topspin in difficult situations is an essential skill that can only be developed with unwavering diligence.
Slice and topspin are hardly the only choices.
If you don't have enough space between the ground and the ball to generate significant upward velocity, why not try hitting the ball flat? That used to be the _standard_ way to hit the ball, you know.
Grab the racquet in a full-continental grip, making contact just after the racquet is in the lowest point in the swing, with the racquet face only slightly open, and swing the racquet forward, mostly horizontally, i.e. with only a slight upward lift.
A flat shot may not make sense if you're taking it near the service line, but if you're near the baseline or deeper you can put a significant amount of pace on the ball without going out of bounds as long as you keep the shot fairly low.
Try to find videos of Rod Laver hitting passing shots. Most approach shots and volleys stay pretty low, so he had plenty of opportunity to handle low balls. Yes, he was famous for his "topspin" backhand, but if you look at the videos his attacking backhands were hit rather flat.