Hi Doulas,
I don’t have any personal experience with an inflatable mattress wedge or its reliability but there are a few comments about inclined sleeping (which is different from systems that can raise the head and foot separately or have a zero gravity position) in this thread and there are some comments in post #57 here from one of the members here about using two wedges as an alternative to an adjustable bed that can raise both the head and feet into a zero gravity position.
I would make sure that your mattress is suitable for an adjustable bed and wouldn’t be damaged with bending and I would also be cautious about using a wedge that didn’t provide even and continuous support under the mattress (see this picture) but these concerns aside I think it can certainly be an economical alternative an adjustable bed for elevating the head and/or feet.
Phoenix