I think that the 1,000 year thing was just hyperbole and the Sarlaac doesn't literally take 1,000 years to digest its prey. It's not like Jabba and his gang were zoologists or anything like that. The Sarlaac probably does have a very slow digestive system that local lore exaggerated to 1,000 years and Jabba didn't know, nor cared, enough that it wasn't true, it just sounded good and made being fed to the Sarlaac sound appropriately terrifying.
As far as somehow keeping its prey alive while slowly digesting it, that makes 0 biological sense to me. Why would any organism waste energy in keeping its prey alive while digesting it? It becomes a 0 sum game, or close to it, spending extra energy to get energy. Unless things like banthas, krayt dragons, & "Goros" are its natural prey that it evolved to subsist on and they're highly, highly nutritious, the Sarlaac would likely be spending as much energy, or even more, in keeping its prey than what it's going to get from it. It's sort of like charging your phone using a trickle charge while still using it.