Hi moga, welcome to The Mattress Underground 
Sorry to hear you’ve so many “failures” in finding a suitable mattress. Good to hear that you have read “various reviews and guides” here on TMU …just in case you missed any of those…the best articles when Mattress shopping are: theMattress Shopping Tutorial, Mattress Specifications You Need To Knowand the Mattress Durability Guidelines The guides in these readings prompt consumers to find out the quality and specs of each of the components in the mattress which is the best safeguard in buying a mattress that does not develop any “valleys” down the road and it does not change the initial comfort/support experience over time. Mandy of the mattresses you tried in the last 6 months are rasing quite a few red flags in terms of quality of the components.
In addition to this, remember that a mattress is unique to you based on your stats(height, BMI, sleeping position(s) and any underlying health conditions) - thanks for providing these - and PPP(Posture and alignment, Pressure relief, and Personal preferences). Since you are a combo side/stomach and occasional back sleeper and your partner is a back sleeper, and having lower BMIs your task may be a bit more difficult to deal with than someone who sleeps in one position since the different positions have different basic requirements. You can read about this In This Article on stats and sleeping position. As Phoenix says:
" You can find much of the relevant information you would need in the sections about sleeping style, preferences, and statistics along withputting the layers together and the page on tips and tricks here ) that can give you some general concepts, guidelines, and insights about the effects of different body types, sleeping styles, and mattress designs for different people but this is only generic and not specific to any particular person so you would need to adapt them to your particular body type, sleeping positions, and circumstances. (May be more complex than you really need to know.)"
Your experience with the (14") Helix Midnight luxe is not uncommon. Also, at your low BMI’s you don’t really need such a thick mattress. This may complicate things a bit as every layer affects all other layers and you might have too many variables to take into consideration. this mattress is comprised of memory foam, gel foam and polyfoam. Memory foam by its nature conforms to a sleepers’ body but can also wear prematurely, as you have seen, causing ‘valleys’ or ‘divots’ which can cause aches, joint and muscle pain and even alignment issues.
You can read several forum discussions on the Helix Midnight Luxe here More recently though (9 months ago) there were some consumers complaining about indentations after 2 weeks of use of Helix Midnight Luxe and you’d want to check that they did not change the foam densities and that each component meets the durability guidelines mentioned above. See post ##1 here as well. As a side sleeper, you definitely want a durable and comfortable support layer on whichever mattress you decide to go with. something using good “firm” support with enough comfort materials on top that are adequate to allow for conformation of your hips and shoulders tends to work well for a combo side/prone sleeper.
The Aurora is also a foam mattress, as is the Sealy and likely the Sleep Science. I would say this is a mattress type you can cross off your list. Since most of the major retailers are less than transparent regarding the exact specifications for their products, and using a less expensive, lower density foam could show wear very quickly as you‘ve experienced
Since every individual has their own specific needs in a sleep system, we don’t really make suggestions, but I will say that since you’ve already invested time ordering, testing and returning mattresses, it would be ideal if you can could go to some local showrooms, and test different mattress builds types - like an all latex, foam, latex hybrid with different comfort levels in the layers, etc. to see what you both prefer. You may have just not found the correct combined latex ‘feel’, or you might more prefer a hybrid with latex, coil layer and possibly a quality foam that meets the durability guidelines linked above - I wouldn’t try any mattress for which you can’t find the specifications you need to know to ensure they meet the durability guidelines. … this will help you ‘cross off’ any mattress types you definitely want to avoid, thereby shortening your list.
Once you have determined a type of mattress and mutually approved comfort level, any of the Trusted Members of the site can help you to choose a mattress if you provide them with a little info about you and your partners’ history with mattresses and your preferences, as they will be likely familiar with the other mattresses you have tried and can help you ‘zero in’ on a mattress which will be suitable for you both, and have generous exchange/return policies if you need to ‘tweak’ your build. You can submit a body profile for Trusted Member Custom Sleep Technology for one example.
I wish you Success on your new mattress journey!
~ Basilio