Hi thevelvetrope, and welcome to the Mattress Underground 
Like many other people - I bought a very expensive mattress (Healthy Life Mattress Firm | Bedbuyerâ„¢ Review (in 2021) 2) which gave out after 5 years. Discovered this forum, embarked on mattress surgery, and after numerous experiments I am very very close to the end but was looking for some advice about where to go next.
Me:
• 5’4", side sleeper, 85kg/187pounds
I took a look at the Healthy Life firm, and I’m actually surprised it lasted so long! The mattrsss is almost 14 inches, and has multiple layers of gel or poly foam and memory foam with no details on density or thickness. The coil layer does not specify gauge or coil count…the latex specified neither thickness or ILD…we always caution about products from the ‘big brands’ as they tend to lack any transparency on composition or density, and use empty terms like ‘premium comfort flexible foam’ to describe materials, which can hide inexpensive
Less dense foams which are liable to ‘break down’ as you have found.
Before looking at your build, though you have been ‘lurking’, let’s’ look at the basics. First, of course, I’m sure you know every sleeper is unique and any mattress you finally choose will be based on your Stats (height, BMI, sleeping position(s) and underlying health issues) - well done providing those, you HAVE been reading the forum! - and your PPP (Posture & alignment, Pressure relief, and Personal preferences). As you can see in theMattress Specifications You Need To Know compared to the Mattress Durability Guidelines latex is one of the most durable materials you can have in a mattress. Also, there is quite a difference in ‘feel’ between Talalay and Dunlop latex: Talalay is ‘bouncier than Dunlop- think of the difference between angel food cake and pound cake…though I’m not sure about the availability of either in Australia- we have had a number of supply chain issues and disruptions over the last few years. Regarding foam, we suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support.
Current build (bottom to top):
• 20cm (~8-inches) pocket coil springs - left the gusset and very thin layer of foam that was glued on, on an Eva timber frame (I have a queen mattress, but a double bed frame to avoid the sides/edges sticking out so there’s less mattress area over the slats than normal)
• 3-inch medium latex layer from Dawn latex mattress (ILD unknown but says density 70 on label)
• 1-inch soft latex layer from Dawn (density 60)
• Cotton/wool underlay
• Currently no mattress encasement but it’s on the way…
As a higher BMI sleeper, if your top latex comfort layer is too soft, you may end up ‘bottoming out’ on the layer below, which can cause aches or numbness as you have found, and even alignment issues over time. When you switched the medium firm layer to the ‘less firm’ side you felt more support but still too firm; possibly a plusher ILD topper might solve this, though it’s a subjective choice. You have found the classic drawback to DIY projects: while they can allow a great degree of personal customization, it can take a bit of time, effort, and consideration to switch layers around to get the right feel…unfortunately there’s no way around that.
Current experiment - flipped the 3 inch medium layer over to the other side (as top was firmer) and this was a game changer! Tried this on its own and support was perfect but too firm (woke up numb/sore in spots). Last few nights I’ve put the 1 inch soft layer over it and it’s so so close but still feels not plush enough, however it also feels on the verge of not being enough support.
So question is - suggestions about where I go from here? I was thinking about getting a <1 inch wool topper maybe? Do I bother trying to put the memory foam back in the mix? I’m worried adding more stuff will affect the support…
I also have a ball fibre mattress topper available but i’m assuming synthetic over latex is not a good way to go? Any ideas appreciated!!
Your latest build sounds good support wise, you just need to ‘dial in’ the comfort level. The Eva Timber frame looks well-constructed of birch plywood and American oak veneer, with good center support and spaces of 3" between slats. The wool topper could well do the trick; I wouldn’t bother with the memory foam if you cannot find the density, or you could use it for a transition layer between the comfort and support layers if you think it still has some ‘life’ in it. The ball fiber topper could also work, it’s polyester fiber balls in a cotton cover; If you do not mind synthetic materials, and have no allergic reaction to them, there’s no practical reason not to use it if it feels comfortable…you want a softer feel but need a firm support layer, so a less thick plush topper or top comfort layer that doesn’t allow you to ‘sink’ to far is the goal, and unfortunately you are the only one who can decide that….you are almost there! You just need to decide on the finishing touch for your top comfort layer; and please let us know your experience once you have had some time on it!
~ Basilio