Hi Alain.
Welcome to our Mattress Forum. 
All the way from Germany it seems.
It sounds like you have quite the odd sleeping setup and I understand your urgency in wanting to purchase something new to get away from the inexpensive, saggy number of mattresses you’re sleeping on now – as well as avoiding sinking too far into your topper and overheating.
If you haven’t done so already, even though retrospectively … I’d still read up a little on the properties of memory foam. It tends to soften with heat and pressure (hence the “sinking in” feeling) which can be great for pressure relief and movement isolation, but also is not as much open celled foam and can “sleep hot” for those who are sensitive to this.
Other resources for sleeping hot that you may find useful moving forward to avoid overheating…the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here more about tracking mattress temperature regulation issues potential causes IN Post #2 here (at least to the degree possible for a specific mattress) and the posts it links to that may be helpful. You may also be interested in the information in post #29 about temperature regulation and mattress microclimate.
Side sleepers have a “curvier” sleeping profile, with more recessed areas needing support, but also the added factor of relieving pressure on the “bonier” points of the body (hips and shoulders). Side sleepers need a deeper cradle to relieve pressure so the gaps are filled in and help spread the weight over the mattress. In other words, they need to “sink IN” to the comfort layers more. This means that the comfort layers generally need to be a little softer and/or thicker than the other 2 positions. If you’d like to know more about the support and comfort needs for your sleeping profile, you can read more here.
It looks like Ravensberger’s 4 variations of the cold foam beds are made of Polyfoam which is sculpted to allow for 7 distinct zoning and conformation. I am guessing that they are calling it “cold” due to the channels that would allow for some ventilation, however I do agree with you that their all latex or the Comfort spring core mattress (not the classic that has Memory foam) would be better in terms of temperature regulation. One unknown variable here is the cover fabric and/or quilting material and you may wish to find more details about that. If you are considering any of the 4 cold foams models you will also want to find out the densities of the foams used (to assess durability) and the IFD to assess comfort/support a little better.
Generally, Latex is the most breathable of all of the foams and is a great option for people who tend to sleep hot, and it has comparable pressure relieving qualities to memory foam as well due to its ability to form itself to the exact shape of your body profile while you are sleeping and to do this instantly as you change positions. I’m not sure how familiar you are with latex, or how much research you’ve done on it, but you can learn more about latex as a comfort layer in [URL=Mattress comfort layers - Latex foam - The Mattress Underground]this article[URL].
Also, the more you sink “in” to a mattress (versus sleeping “on” it), the more potential you have for sleeping warmer, and the less of your body’s surface area is exposed for heat exchange.
All the way around with all information I gathered it looks like the H3 Comfort spring has the qualities you want, but you still have more legwork to do. Have you had a chance to test it?
I hope your research on the site has included following the guidelines in the mattress shopping tutorial as well. Please let me know if you have any other questions arise.
Phoenix