Hi Vicente,
Welcome to the forum

.
Thanks for the post, and sorry to hear about the Luma mattress not working out. So your BMI is 25.8, pretty low actually, and your girl or partners BMI is just over 20, also quite low.
Luma Sleep, like all of our
trusted mattress members
is very transparent with their materials. They only have 2 coil bases, the bolsa and quantum elite (combi zone), and the quantum is clearly the firmer unit, why didn't you just get the Hybrid Slumber system? Also, the 28 ILD Talalay is too soft?
Of course, you can exchange the 28ild, and get a 36ild with the Luma Sleep Comfort Exchange program, but I'm not sure what your issue is. Are you saying that when you and your partner are lying flat on the mattress you are "sinking" in toward each other? With 4 inches of latex and new L&P coils system? Maybe I am misunderstanding?
find a mattress that is conforming, somewhere between medium/firm, but that does not feel like my partner and I "sink" when we are sitting or laying in bed beside each other, as in the weight of one person drags the other person in. I am 165 lbs and 5 foot 7 and my girl is 133 lbs and same height as me. We are mostly side and back sleepers.
This is why it is really hard for you and companies like Luma Sleep. There are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific recommendation about which mattress or combination of materials and components or which type of mattress would be the best "match" for you.
You guys are Side / back sleepers, and low bmi and want a medium/firm mattress - its very understandable that a lot of our member companies would not think you would sink in toward each other with - Bolsa + 2" 75k density Dunlop latex, + 2" of 28 ild Talalay. I mean any people sitting on the side of the bed, that seems normal to sink in toward each other.
Of course, you can find and ask questions with our
Mattress Expert members
, but are you looking for advice to help you with Luma or are you going to return the Luma and start shopping for your third mattress?
The only way to know for certain whether any specific mattress will be a suitable choice that provides you with both the right comfort/pressure relief and support/alignment will be based on your own careful testing or your own personal experience when sleeping. Different people can have very different needs and preferences in a mattress in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences). Also what is considered firm for one person, can be soft for another.
I think in the future you can be quite clear that you need the base support of your mattress to be FIRM, this "preference" is clearly very very important to communicate when you do future mattress shopping. What part of Canada are you in?
Thanks for the post, let us know what way you are leaning and we can try to help.
Sensei