Recommendation for an elderly parent

Dear John Ryan, I am exploring an upgrade of a mattress for my father.  He is very elderly and frail, weighs about 80kg and is a back sleeper.  The necessity has arisen after a few unfortunate falls recently, and the consequent need for a higher bed to allow for safer transitions both on and off it.  He has a very low Hulsa bed with its original foam mattress.  I have so far tried a high quality wool topper which is much better and raises the mattress depth to approximately 28cm.  I think this is about the lowest we should aim for in terms of depth, but ideally go for something deeper.  However, the mattress still sags and I assume this is because the underlying mattress is well past its replacement date.  The Hulsa foam mattress allowed the bed to be raised in various ways, but I think this will have to be sacrificed for depth and comfort.  I am considering a Simba 34cm thick mattress, I have their topper on my own old Vispring mattress and have found it very comfortable, but my brother has one of your mattresses and says he has found it excellent.  So I thought I would write to ask what your opinion and advice might be.