Construction of drywall with existing wooden posts

Hello everyone,

I would like to install a drywall partition in the attic.
As part of the roof structure, or rather to transfer forces from the roof structure, four KVH posts measuring 10x10 cm have been placed on the unfinished floor (with a vertically resting beam on top). So I already have part of the stud framework.

Good sound insulation between the rooms is important to me. Furthermore, on one side of the wall, I would like it to be flush with the already plastered lime sand brick wall. According to the caliper, that is 21 mm.
With these parameters, I have initially come up with this construction for myself.
I would fill the cavity with SteicoFlex wood fiber, or is there a more sensible recommendation?

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Between the 10x10 posts, I would add more squared timbers and attach them to the existing upper crossbeam with brackets. For sound insulation reasons, according to research here in the forum, it is recommended to install a stud framework on the unfinished floor, correct? Do I then need a horizontal (decoupled) squared timber on the unfinished floor between the 10x10 posts? Looking at how it is usually done from scratch, probably yes, right? There is already screed, but the screed installer made a separation cut between the rooms. If it makes sense, I would remove the screed, too. A few centimeters of insulation lie underneath, of course.

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I would be very thankful for tips.

Best regards

On the manufacturer's pages for Rigips boards, there is technical information on single and double boarding, board thicknesses, white, green, and blue boards, and the corresponding sound insulation values, just take a look

All right. Can you also say something about the topic of screed between the posts, or placing crossbeams on the screed?

I wouldn't do anything to the screed because of the dismantling option, only separate with felt underlays from the wood