Hi Digger,
I think that your spare bedroom and mattress is getting far too much use 
Unfortunately … I can’t feel what you feel and I can’t see you on the mattress and since there isn’t a formula or “theory at a distance” that can choose a mattress design for someone else, any guesses I make won’t be nearly as accurate at assessing the suitability of a mattress as your own careful and objective testing or your actual sleeping experience (see mattress firmness/comfort levels in post #2 here).
The key to alignment with any mattress (which is the most likely cause of lower back pain to the degree that a mattress contributes to the back pain) isn’t necessarily how much you sink in but how evenly you sink in which determines how well you spine is aligned and part of this is determined by the weight distribution of each person and the surface area that is in contact with the mattress and of course the sleeping positions. While it’s true that heavier people generally need firmer comfort and support layers … there are so many different body types and preferences that this certainly isn’t always the case.
Less curvy body types also don’t have the recessed gaps that need to be “filled in” and supported as much so they can also do well with firmer materials.
For you … the one thing that seems clear to me is that 4" of soft latex in your comfort layer is clearly too thick/soft for you to maintain good alignment.
For him … there is some history of back pain already that may not be connected to the mattress and I don’t have any clear sense of what he may do better with outside of the “pointer” based on his experience that a softer thicker comfort layer seems to be better than a slightly thinner firmer comfort layer. Even though he seems to prefer more softness … I would be tempted to try out a thinner soft comfort layer for him as well since this seems to be closer to your old mattress (a thin layer of softer memory foam over a firmer mattress) and the most common cause of back pain (but not the only one) is a mattress with comfort layers that are too thick and soft.
A change from 3" comfort layer of latex in the medium range to a 4" comfort layer in the soft range is a fairly big change and may also have “jumped over” the thickness and firmness that is best for either one or both of you.
I should also mention that part of the issue may be the layers that are underneath the removable topper which would have at least two layers (a latex core is only 6" thick) so the top 2" or so of the mattress itself and their ILD may also be playing a role in all of this and would affect how the topper feels and performs.
Now that you have personal experience and a reference point for mattresses that you don’t do well with, if I was in your shoes I would be tempted to do some very careful and objective testing in the showroom (not just based on how a mattress “feels”) to find the mattress that is the best match for you based on actual experience and I would pay very careful attention to whether a mattress you are testing seems “familiar” or reminds you of the feel and performance of the mattress that each of you don’t do well with. I don’t know of any way other than your own personal testing to choose a mattress that is the best match for you unless you have a specific reference point of a mattress with a known design that your experience indicates you do well with.
Based on the complexity and uncertainty of your situation … I would be hesitant to “lock in” anything that you may not be happy with unless it was clear to you that the only thing necessary was adding a topper because the mattress was too firm.
I would also consider trying to sleep on just the mattress with your old memory foam topper to see what the differences in your experiences are which may also provide a useful “pointer”.
Phoenix