Hi pixbroker,
I added the images to the reply where you originally linked them.
I think SleepDeprived’s suggestion is a good one if for no other reason than to rule out one possible cause. Sometimes the symptoms of an alignment issue can take a while to become noticeable (the body may be able to deal with it for a short time without producing “symptoms” but not over the course of hours) so it’s always worth trying a solid surface just to confirm the main underlying cause is the mattress and not the support underneath it.
Post #4 here also has some ideas that can sometimes help a sagging mattress at least partially or temporarily.
Phoenix