Hi blue,
First to the part of your question about mattress testing. The goal is always to test specifically for pressure relief (which is generally easier) and for alignment completely relaxed in all your sleeping positions (for at least 15 minutes). Each sleeping position can have different “issues” (for example side sleeping may have pressure issues and back or stomach sleeping may have alignment issues) and each needs to be tested separately. The most important part of this is to listen to the more subtle cues from your body because any discomfort you feel in a showroom can get worse over the course of the night because time can make both pressure issues and alignment issues worse. There are some guidelines for testing for pressure relief here and for testing for alignment here and in post #11 here. Listen carefully to what your body is telling you and how relaxed and how well you can completely “let go” with no muscle tension on the mattress rather than focusing only on a subjective and less meaningful sense of “comfort”.
When you test mattresses with the specific intent of identifying pressure relief and posture/alignment … it will be much more accurate than just testing for more subjective ideas of comfort and if you tend to err on the side of firmness (bearing in mind that all mattresses will soften to differing degrees in the first few weeks) it increases the odds greatly of making a good choice and that any “fine tuning” will be less and involve making the comfort layer softer which can be done with a mattress pad, protector, or topper without having to exchange the mattress itself.
Rather than a brand … it’s usually a better idea to identify a material and the thickness and softness of that material that has the best odds of matching your needs (pressure relief and posture/alignment) and preferences. Prices will vary between manufacturers and “brands” but the material itself is what is really important so if you know what you want from a material point of view then you can make meaningful comparisons between brands. Post #4 here has some of the better sources of different toppers I’m aware of although of course it is by no means a complete list.
When a mattress is too firm and you are looking to make it softer … it can be very difficult to make specific suggestions for someone else because they can’t feel exactly what you are feeling on the mattress and there are many variables involved in terms of the thickness and softness that may work best for any particular person. An example is your own circumstances where a topper that is too soft can feel too firm while for someone else it could be the opposite issue where the upper layers themselves are too firm. Some rough guidelines for what topper may work best for different people are in post #8 here.
In your case … you already have a comfort layer that is 4" (the 1" in the mattress and the additional 3" topper) which means that it is already on the thick side and the issue is not so much that it is too firm but that it’s too soft which is “allowing” some of the underlying firmness to come through because your heavier weights are sinking through the top layers more easily… Because it is already quite thick … it would be more important to choose the thinnest possible topper if you were considering using it in addition to what you have already.
If you were replacing the topper you have … my tendency would be to try a 3" layer in the range of about 24 ILD which would allow you to sink through it less than the 14 ILD you currently have and has better odds of isolating you from the firmness of the layers below. Even this would be on the soft side for your weights but I think this could be a good idea with fibromyalgia. The very lowest I would go is 19 ILD but this may also have some risk of allowing you to “go through it” more.
If you want to keep your current topper … then it would be a matter of deciding how much more you need to isolate you from the support layers (a touch to a little … a little to a fair bit … or a fair bit to a lot). I certainly wouldn’t go more than 2" (probably in the 24 - 28 ILD range) and I would put this underneath your current topper so you have the same “very soft” 3" surface layer and then an additional 2" underneath this (to help with the transition between the soft topper and the firm support layer) all over the extra 1" of 19 ILD. In effect you would be building a “nest” with different layers 
If you think that “just a touch to a little” would be enough for you and taking into account that thinner is better for alignment … then I would choose an additional 1" and also put it under the current topper. Once you are good with what you have … then you can always buy a new topper cover to keep them both together if you don’t want two separate toppers.
I know this is not an exact science but hopefully this can give you a good sense of which may work best for you and some of the tradeoffs involved (choosing between softness and thickness and the risk of alignment issues).
Phoenix