Are you sure, you only need alt-hold?
I think indoor basic you need optical flow sensor to hold xy position to prevent drifting, optical flow sensor needs a lidar height sensor. You may need front and rear lidar sensors for object avoidance. This product probably the benchmark.
Simultaneous Localization and Mapping got nothing to do with height, more for computer navigation.
So, I think firstly you need a loiter flyable drone, hover and lock to position. If you are using for pilot flying, side, rear and front liar sensors to assist the pilot from crashing. If non-pilot flying, a onboard companion computer is needed. Maybe this is something you like to understand further.