Hi Jezo,
A topper is most effective for softening the surface of a mattress that is too firm and from the sounds of it your mattress has become too soft and lost it’s support … at least under your pelvis/hips (which is usually the case when you are waking up with back issues). This is much more difficult to “fix” with a topper because the topper will still sink down into the soft spots below it. At best … it may be a partial or temporary fix.
If the soft spot is not too bad … then a topper may even it out a bit and help to some degree although it’s still not ideal and in some cases it could become worse (depending on the condition of your mattress and on your body type and sleeping style and the type and thickness of topper). A thicker tipper will even out the soft spot more but can create it’s own problems by still allowing your heavier parts to sink in too far. A thinner topper will even out the soft spot less but then the soft spot can still allow your heavier parts to sink down too far. Either way is “risky” with a mattress that has become too soft. You can read more about some of the difficulties involved with “fixing” a mattress that is too soft in post #4 here.
If you do decide to experiment with toppers … the first thing to do is to lie on fast and slow response layers in a store to get a clear sense of which you prefer. Memory foam is a slow response material and polyfoam and latex are fast response materials. The choice between these is a matter of personal preference and you can see some of the pros and cons of memory foam here and the pros and cons of latex here. Polyfoam is also a fast response material and has the benefit of being much less costly than either good quality memory foam or latex so that a topper that doesn’t work is less costly.
Beyond this … there are so many variables (your body type, sleeping style, the layering and condition of your mattress, and the type of topper) that only experimentation will really answer if a topper will help and if so which type and how much effect it will have. This means that I would buy from a big box store or other source that allows you to get a refund if an experiment with a particular topper doesn’t work out for you.
I would suggest something in the range of 2" - 3" and since you are only looking for it to last for a couple of years then durability (and the cost of more durable materials) is not as big an issue. Bear in mind that more than other types of materials … memory foam tends to be a material that is very different from fast response materials and tends to be a “love it or hate it” material.
A few examples …
Something like this at Costco or like this at Sams club may be worth considering for a slow response memory foam option (they’re both 5 lb which is good quality and you won’t “go through” it as much into the soft spot underneath).
This one at WalMart may be worth considering for a latex “fast response” experiment.
Make sure you check the refund policy for each on before you buy it so that your experimentation doesn’t become too costly.
Phoenix