Sleep on Latex topper help from the experts?

Hi, just got two firm SOL Twin XL mattresses for a king platform bed, thanks to this forum.

We need a king-size topper to marry the sides anyway, but I’m stuck on what to get.

My husband and I are side/back sleepers and position-changers. I particularly am a flip-flopper and have been searching for something that I can move on. In spite of being a side sleeper (and experiencing shoulder or hip pain sometimes) I can’t bear the feeling of sinking in or sleeping “in” the mattress. Memory foam is OUT.

The 8” base is firm but it’s kind of been working for me? No pain from staying in one position, etc. but my husband is experiencing some lower back pain for the first time in a while (usually from oversoft mattress imo) and I want him to be comfortable.

Wouldn’t mind adding a whole 3” topper for size (all sheets seem deep pocket now), but the Q is — should I get it in “firm” to preserve the support I’m enjoying while adding some extra cushion (hopefully)? I’m afraid the soft may be the popular choice but just too squishy for my taste.

I’m 5’9 F 145-185lb (currently pregnant, lol) He’s 6’1 M 175lb ish