Hi restoms,
Thanks for taking the time to share your comments and feedback and for letting us know what you ended up deciding.
You certainly made a great quality/value choice although I’m sorry to hear that the firmness level you chose isn’t working out as well as you hoped for. The good news of course is that SleeponLatex has a great return exchange policy that can minimize the risk of making a choice that isn’t a great “match” for you in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences).
This would be the norm with any DIY mattress because you are buying individual components rather than an actual mattress. Having said that … mattress warranties only cover manufacturing defects and they don’t cover the gradual (or more rapid in the case of lower quality comfort layers) loss of comfort and support that comes from foam softening or breakdown that is the main reason that most people will need to replace their mattress so warranties have little to do with the durability or useful life of a mattress or how long it may be until you need to replace it. If there is an actual defect in the materials it will usually show up early in the life of the mattress (and most reliable suppliers will replace components that are clearly defective) but knowing the quality and durability of the materials in your mattress is always a much more reliable way to assess the durability and useful life of a mattress than the length of a warranty. There is more about mattress warranties in post #174 here.
There are no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.
You can see an example in this topic where their firmest latex mattress felt to soft for one of the members here.
I’m looking forward to finding out how the medium works out for you 
Phoenix