I normally post the standard note to make sure you’ve read Phoenix’s post on how to select a mattress that’s right for you.
If you’re going to make a DIY mattress, it’s least risky to start with a mattress you already like and matches your PPP, and try to duplicate it. Phoenix has tended to warn people about doing a DIY mattress, as it will require some experimentation, and may end up being more costly than getting a pre built mattress. If you’re doing a DIY because you enjoy it, or want to experiment, then go for it.
From my personal perspective, I have both a Dunlop mattress and a 3" wool topper, and I can say that a 6" core of 4.98 pcf (about 80 kg per m3) (the softest you identified that is available) with a 3" wool topper on it would be incredibly too firm for me. I’m male, 5’9"’ about 185 lbs.
My top layer is 3" of wool, below which is 6" of 65 kg per m3 (about 4.05 pcf) soft Dunlop, below which is a firm 3" Dunlop base, and even that I find potentially slightly too firm for ‘perfect’ pressure relief, although my posture/alignment seems fairly good.