Brooke & Wilde Elite Mattress too soft & showing issues of sagging already

I have recently purchased a Brooke and Wilde Kingsize hybrid mattress in soft brookandwilde.com/the-elite-mattress/ it is on a sprung slatted base.

Neither me nor my wife are overweight, in fact, quite the opposite. When in the bed alone if in the middle of the bed the mattress is fine but if I move to one side it becomes quite hard.

When the two of us are in the bed the softness is halved and it is uncomfortable for both of us to sleep on.

Is this because the mattress is soft and tends to be less able to support two bodies than if it were firm or is it down to the slatted base? If I replace the base with a sprung divan from you will it help this issue?

I notice that not many companies offer soft mattresses and wonder if the reason for this is that a firmer mattress offers a softer sleep experience due to not compressing so much?