Hi Redfox!
Thanks for your question and congratulations on your new mattress! Welcome all the way from Australia! We love the Aussie unique strain of healthy humor (with and without a good night sleep) 
[quote]I had purchased a new bed frame/foundation but used 10x wooden slats from my old frame (12 years old). These wooden slats were Pine wood, 3cm thick and 9cm wide. The gaps between these slats were around 8.5cm - 10.5cm. I slept on this for a week and it was comfy, it was the right firmness for support with plush for comfort.
However, I realised that these gaps may be too wide and reduce the lifespan of my new mattress. So I went and bought brand new slats. 14x Pine wooden slats 3cm x 9cm. Now the gaps are 3.5cm - 4.5cm (around 1.6"). Sleeping on it for the past 3 nights I feel like the firmness has increased and gotten less comfortable than previously with 10x slats.[/quote]
I appreciate the background information. I entirely understand wanting to use the correct base for your new mattress and get the most comfort and longest life out of your new purchase!
You have a latex mattress and you are using pine slats (3cmx9cm). I realize there is not a whole lot you can do with your frame, since the slats are currently glued in place. That said, if you feel the mattress is too firm, you could try with 7.6 cm (3") gaps maximum for better comfort.
I took a look and can see that the mattress is 100% natural latex. All foam materials will soften over time but natural latex, in general, is the most durable of all the foam materials so it will soften less and more gradually than other types of foam materials as it is denser, more resilient, and more supportive than its blended or synthetic latex counterparts.
The softness of a foam material will also affect its durability because softer layers are less durable than firmer layers that use the same type of material, and foam materials will also soften and break down faster for those that are in higher weight ranges than for those that are in lower weight ranges.
There will be a break-in and adjustment period for any new mattress or sleeping system as the mattress loses any of its “false firmness” and the cover stretches and loosens a little and the materials settle and your body gets used to a sleeping surface that is different from what it is used to (Learn more about that in here).
You may have already come across this resource, but it never hurts to check out/compare the specifications of the mattress you have purchased with our Mattress Durability Guidelines.
With 6" mattress thickness (15 cm) I think you are just on the threshold of sleeping well and not so well on this mattress mainly because the mattress is a bit too thin to allow for a right comfort/support balance for a BMI of 25.3 kg/m2 (73kg, 170cm) which leads me to believe that you might do well to add a topper. If I could comfortably afford it there is no doubt I would go in the direction of adding more thickness to the mattress (see post #14 here ) and also check that then the topper/mattress combo is a good “match” for your weight range in terms of PPP, but I would wait for the adjustment period before I would make that determination.
The first thing to do is always to check with the mattress manufacturer you’ve purchased and ask (in writing) of their opinion and permission to make sure that the foundation is appropriate for the new slat arrangement and doesn’t void your warranty (the difference would be between .5 – 1.5 cm which is not too much and I expect that a good manufacturer would not void the warranty). But you need this in writing if things don’t go as well as you expect with the durability of the product.
I agree that 3cm seems close and IMO not that necessary. Most manufacturers recommend 3" (7.62 cm) or preferably less for latex. You would also want to compare this with the width of the slats themselves. I would consider you also check the recommended slat width with your mattress manufacturer if that information is available.
I hope this helps offer some clarity regarding the best base for your new mattress!
Jethro