Hi Anna,
You seem to have gone from one extreme to another and somewhat “overcompensated” for your experience with the Simmons 
While I can’t tell you what would be “best” for you because either of your choices may work and only you can know that … I can at least tell you the pros and cons of each of your options and how I would choose so that you can make the choice that is best for you.
Now that you have some experience sleeping on the Elsfjord … I think you would be in a good position to evaluate any difference in the Edsele with personal testing and the first thing I would do is test the Edsele carefully and as objectively as you can using the testing guidelines in post #1 here (and making sure that you spent at least 15 minutes on it after you have reached a “pre sleep” state of relaxation). I would make sure that you listen carefully to the more subtle cues from your body for any hint of “familiarity” about a mattress that is too firm for you in term of pressure relief.
If you are confident that the Edsele is suitable for you in terms of PPP (Pressure relief, Posture and alignment, and Personal preferences) then I would lean towards an exchange because the Edsele is a much higher quality and higher performance mattress than the Elsfjord and then you wouldn’t need a topper at all (although you would still have that option if it turned out to be too firm as well).
The other alternative as you mentioned is adding a topper to the Elsfjord and in this case I would use the topper guidelines that are in post #2 here and the posts it links to (especially post #8 here).
I don’t think a comforter would be a good indication of how a latex or foam topper would work for you (they would be completely different) but based on your description and on your body type I think a 2" topper would probably have the best odds for you (especially if this seems to agree with your experience about needing a “little to a fair bit”)
The advantage of this option is that if you choose a topper that doesn’t work well then you could just exchange the topper for another one (assuming that it can be returned) and the risk would be lower than if you exchanged for the Edsele and for some reason it turned out to be too soft for you (which isn’t likely) because it’s not really possible to “fix” a mattress that is too soft because you would need to remove some of the excess softness rather than just adding a topper.
So I would make my decision based on your level of confidence in testing the Edsele. I would lean in that direction if you were confident it would work because you would end up with a much higher quality mattress which had about 1.5" more of a higher quality latex (roughly the thickness of a topper you would be adding anyway to the Elsfjord) and a higher quality cover that includes wool.
If you were still uncertain from your testing … then the topper direction would give you more options and making a change from your initial topper choice would only involve exchanging for another topper. You could choose the replacement topper based on your actual experiences on the Elsfjord / topper combination which would probably give you a very good indication of what a replacement topper should be if your first choice isn’t suitable. This way … if you buy a topper that can be exchanged … you would only be out the cost of returning it and would still have more options available.
Phoenix