Hi aladrieth.
Kudos!
The mattress you describe is similar to the Nature’s Rest Firm mattress. If this is indeed the mattress you tested, I am wondering if the shop you visited is one of Nature’s Rest authorized dealers in Canada. because as far as I am aware their authorized dealers are mostly around the US east coast. Other than this, while the price is a little steep this mattress uses good/quality materials which do not raise any red flags in terms of durability. Generally, it is less important to know the density and ILD for an all-latex mattress (latex is the most durable of all types of foams especially in the firm and medium comfort options. It is best to base your decision on how it felt you in the shop in terms of PPP (Posture and alignment, Pressure Relief, and Personal preferences) and then run everything by your personal value equation and what is important to you
There is nothing wrong with a modular design, it has the advantage of simply replacing one module if your sleeping needs or preferences change or if a layer wears out. You can even rotate layers to allow them to rest and extend their useful life. If there is some slight shifting of the modules over time it’s easy to slide them back in place once you notice it. The key, like with every mattress you are considering, is to find all the mattress specifications you need to know so you can compare the quality of the materials and components to the mattress durability guidelines here. I have not spoken with Martin (the owner of Halstead) for quite a while now as they are not a Member of TMU but generally, they are good mattress people focusing on quality/value products and who strive to satisfy their customers.
Another option you may want to consider a DIY under the guidance of an experienced & knowledgeable retailer or manufacturer that can approximate the feel of the mattress you like (if they are familiar with it)
Hopefully, someone that has experience with either mattress will see your post and chime in with some feedback.
Phoenix