Mattress got firmer with flat ply

Hello,
I have a super king size pocketed sprung type - (do not flip one). When i purchased the bed frame, it came with curved slats which made the mattress feels plushy (which i liked very much) but after few days only we started to suffer from back pain as the mattress dipped in the center.
We replaced the curved slats with flat MDF ply of (15mm) thickness. There is no more dip in the center but the mattress became very firm and uncomfortable.
In this case what should i do to have a plushier and more comfortable feel?
Mattress type: plush - pocketed sprung - 180*200 - no flip
My weight: 85 kg - my wife : 50kg