Hi Alvarez, welcome to The Mattress Underground 
Glad you are here.
Sorry to hear about the deterioration of your mattress – good idea to replace it as mold is definitely not something you want on your bed. As you have already learned, you need to elevate your bed on a platform or base to allow ventilation and air to circulate around and under it to prevent mold.
Whether you are aiming to buy on line or locally first step would be, if you have not already, to find what type of mattress you like in terms of materials used (memory foam, latex, hybrid etc) and then the mattress style you prefer (tight top, pillow top, etc) I’d recommend going to a local store and getting accustomed with how each feels. you can read our Mattress Specifications You Need To Know for any product you are considering. If you can’t find those specs assume that the company has “something to hide” in terms of quality of some or all of mattress components. Once you have the specs then compare them to our Mattress Durability Guidelines to get a better idea of what to look for in a new mattress as outlined in this Mattress Shopping Tutorial for some tips for your search. If you don’t do those initial steps, you’ll run the risk of ending in the same predicament you did with your current “sinus” mattress (I am assuming it is a Zinus which is a very low budget mattress, notorious for using low quality materials and breaking down prematurely) On top of the mold issue you are having with it, as you have discovered the premature break down of foams is will negatively impact your wellbeing and most likely lead to more chronic conditions and sensitivities due to spine misalignment and possible higher VOC content.
While shopping, additionally I’d keep in mind that a new mattress is unique to the sleepers who will be using it. It should be based on your stats (height, BMI, sleeping position(s) and any underlying health issues) and is based on your specific PPP(Posture and alignment, Pressure relief, and Personal preferences) The good news is that both you and your partner are quite similar in style and preferences and you won’t’ need to make up for major weight differentials or sleeping style. Good job in including your and your partners stats, you are both in ‘normal’ BMI range and are combo sleepers, alternating between side and back sleeping, and you have developed some back pain from If you like memory foam products remember that memory foam of more than an inch or so of lower than 4 lbs/cuft is one of the most inexpensive and least durable of mattress materials which can break down very quickly. Talking about materials, have you tried latex or a latex-hybrid product? This type of mattress would be a higher quality mattress that would still fit in the budget you quoted (for a queen size)
As I mentioned above, since you are looking for a mattress which is ‘firm enough to not slouch and cause back pain but soft enough to feel like sleeping on a cloud’, you might want to consider a latex or hybrid latex mattress. Latex is both breathable and provides support, and multiple latex layers can allow customization (such as a softer top comfort layer with a firmer) so that you can experiment in order to readjust your body and get into a more natural and neutrally spinal alignment Support layer beneath, or you could go with a pocketed coil base, which might bring the price down. Since you are in a very humid climate, its important to have breathable layers in the mattress as well as elevate it off the floor with a frame or platform to prevent the mold issue.
This information is older, but the response from Phoenix in poet #2 here discusses some manufacturers in your area.
You can also take a look at theTrusted members of our site and take a look at their online offerings, as these are all excellent in their transparency, construction and customer response.
I wish you well on your mattress buying journey, and we can possibly make some comments on any mattress you are considering going forward.
Basilio