Source bladder here, brilliant being able to get right into the thing and empty it out properly. only ever put water in it.
Empty out, rinse dry out as much as I can then leave it open and upside down on the draining board overnight, stash the hose, coiled up inside it and then just keep it in a dry cupboard.
The freezer thing is a new one on me, but I can see the logic might start doing that it can’t do any harm I suppose, might have trouble explaining it to the missus though…
TBF you don’t have to buy a posh bladder though for the cost of a Camebak or Source bladder you could get 4 or 5 cheap one’s off ebay and just bin em when they get manky…