Hi CT29,
It seems to me that the Sultan Favang would be a good candidate for a topper and I’m not surprised it is quite firm to you with only 5" of foam. Thinner single layer mattresses tend to be firmer and higher weights would make it feel firmer yet as you will compress it more. I’m not sure what the status of the Heggedal is but if you ask them it would be great if you could let us know.
While a topper can be a good choice to improve pressure relief for a mattress that is too firm … it’s can also be risky unless you can test the specific mattress/topper combination in person. Without personal testing … it can sometimes be almost as difficult to choose a suitable topper as it can a mattress in the first place.
The same would hold true for any other changes or components you are considering which can change the feel and performance of your mattress.
I would personally test the combinations you are considering at Ikea so that you can use your own personal testing to make sure that your sleeping system provides you with good PPP (Pressure relief, Posture and alignment, and Personal preferences). This is the most reliable way to know how well it will work for you. Post #1 here has some suggestions to help you test each combination for pressure relief and alignment.
While I would choose the combination that works best for you in your testing in terms of PPP … if you do decide to add a topper to your mattress without testing the specific combination first … then the guidelines here may be helpful.
Phoenix