Hi arndis,
Welcome to the Mattress Forum! … and thanks for the kind comments … I appreciate them 
Most of your comments and questions are somewhat generic and would have too many “it depends” in the answers to provide any specific guidance about what to choose but I can certainly provide some general information that may be helpful in terms of “how” to choose.
There is a lot of misinformation about organic, natural, chemical free, green, and “safe” mattresses and materials throughout the industry but there is more information in post #2 here that can help you sort through and differentiate the more factual information from the marketing information you will encounter about all of these interrelated topics that can help you choose a mattress and materials that meet your criteria in any budget range and that can help answer your questions about “how safe is safe enough for me?”.
If your mattress is still in good condition and there are no soft spots or sagging in the sleeping surface and the only issue is that your mattress is too firm then a topper can certainly be a good solution to add some additional softness and pressure relief to your mattress. If your mattress does have soft spots or sagging (visible or virtual) then a topper won’t be an effective solution and at best the benefits will be temporary or partial. Since your mattress is 12 - 15 years old and considering the addition height may be an issue you would probably be better off replacing your mattress. There is more information about “fixing” a mattress that has lost its comfort and/or support or that has developed soft spots or sagging over the years in post #4 here. If you do decide that a topper would be a good idea in your circumstances then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness of a topper that would have the best chance of success with your mattress, your body type, and your sleeping experience.
The choice of materials or the type of mattress that you choose would be a personal preference but if you like the feel and performance of an all latex mattress more than other materials or types of mattresses then this can certainly be a very good option although as you know this will put you in a higher budget range than many other types of mattresses because latex is one of the more costly mattress materials.
A latex/polyfoam can certainly be a good option that can provide you with many of the benefits of sleeping on latex but in a lower budget range (the support core of a mattress is generally the thickest layer or component of the mattress and polyfoam is much less costly than latex). There is more about a latex/polyfoam hybrid vs an all latex mattress in post #2 here.
An innerspring support core will generally also be less costly than a latex support core (although it may be more than a polyfoam support core) but this would be more of a preference issue along with a budget issue. Some people prefer an innerspring latex hybrid even more than an all latex mattress but your own experience will be the more reliable way for you to know which type of mattress you tend to prefer or that works best for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). There is more about innerspring support cores vs latex support cores in post #28 here.
Only you can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved for anyone to be able to predict which mattress will be the best match for you in terms of PPP based on either specs (yours or a mattress) or “theory at a distance”. The most reliable way to choose a mattress that is a good match for you will be based on your own careful and objective testing … hopefully using the testing guidelines in the tutorial post (see mattress firmness/comfort levels in post #2 here).
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful comparisons between mattresses and there is more about the different ways to choose a mattress and some suggestions that can help minimize the risks involved in each of them in post #2 here.
You’ve probably seen the Ottawa list in posts #2 and #3 here because you seem to be aware of most of the options that are closer to Ottawa.
The better options or possibilities I’m aware of in and around the Montreal area are listed in post #276 here.
Just in case you haven’t read them yet … you can read some of my thoughts about Essentia and some of the misleading claims they make and some forum discussions with them in this thread and this thread and posts #3 and #4 here). Needless to say I would be very cautious here but given your comments about your budget range they will likely be outside your budget anyway.
The mattress shopping tutorial also includes this link to a list of some of the better online options or possibilities I’m aware of that ship across Canada as well if you are open to considering an online purchase.
Phoenix