Hi mnn4532.
Congratulations on your new mattress purchase from Arizona Premium! 
As with all our Trusted Members here I have no doubt that they will not hesitate to help you find the best combo and thickness/softness they have available that works for you. It usually takes a bit of trial and error and a bit of time that the body needs to adjust, but I am glad that you’ve had the foresight to chose something that allows for fine-tuning.
While side sleepers need a deeper and/or softer comfort layer to allow for more contouring and pressure point relief how far each part sinks in has to do with how much weight it carries and the surface area that carries the weight, At your 24 BMI and being a side sleeper It may be that a 14" + mattress with 6" > comfort (3" soft + 3" medium and added pad) is not supportive enough to keep your spine in neutral alignment which would cause the pains that you started to experience after 2 weeks. l generally suggest that any changes to the softness/firmness are done gradually allowing for a little more generous adjustment period (you to the mattress and the mattress to you) The break-in and adjustment period for any new mattress or sleeping system as the mattress loses any of it’s “false firmness” and the cover stretches and loosens a little and the materials settle and your body gets used to a sleeping surface that is different from what it is used to (see post #3 here ). This would typically be a few weeks but it can be shorter or longer depending on the specifics of the person and the mattress (higher density materials can take longer) and it can be surprising to many people how much their sleeping experience can change over the course of the first month or so. Once you are past the initial break-in and adjustment period then any further foam softening or “virtual impressions” will tend to be more gradual and happen over a longer period of time (although this wouldn’t happen so much with latex).
Latex, in general, is the most durable foam material and at your BMI it would not be softening/sagging in any meaningful way with only 2 weeks of use, if anything this it would come from the quilting layers or the base layer under the mattress or in many cases what it is called "softening " is really what I call “virtual impressions” which are the result of choosing a comfort level that is too soft. You can see some more detailed comments about this inpost #2 here .
Over the last two weeks, your body adapted to the new sleeping surface and attempted to find a new balance and compensate for any possible misalignment so I’d recommend that you do a bit more experimenting and perhaps remove some of the softer layers such as the added padding and see if the pains subside. This will give you good hints if the cause of your lumbar pain is too much softness/thickness in comfort layers. Trust your instincts, test specifically for alignment when you are fully relaxed and your muscles have “let go” and as always use the knowledge and guidance of the manufacturer themselves. I agree with you that before this you’d first need to give your foundation a thorough check-up to eliminate this as a possible cause.
The metal frame you own would be sturdy enough to support the weight of the mattress and sleeper(s) on it but it is not clear enough how the wood bed frame combined with the 4" low profile metal frame would work together. I generaly don’t trust two foundations on top of each other in terms of stability or flex but this would certainly allow you to get closer to the total height you want. If possible, I’d try to rule out any issues and I’d lift the wood frame against the wall somehow and try the mattress on the metal bedframe
I’m looking forward to your comments and the results of your experiments once you’ve had the chance to try a few more configurations.
Phoenix