Simba Hybrid mattress too firm - what is the alternative?

We just bought the Simba hybrid mattress and it’s way too hard for me and has given me lower back ache sadly.

I’m 8.5/9 stone (5ft4) most the time and my hubby is 13 stone (6ft).

I like a medium/firm feel mattress and hubby likes soft/medium feel in a bed. What would be the best super king bed for us? Can’t try any because of covid and need mattress advice!