Old building new window internal connection?

Hello,

We have installed a new window in the kitchen of our old building from 1989 (uninsulated 30 cm brick wall). It was sealed all around with a window foam (approved for installation according to RAL). The transition between the wall and the window frame on the inside was additionally sealed with an acrylic joint. I am aware that this does not meet new construction standards. My question:

On the shelf area visible in the photos (previously there were tiles here), a wood decor board (backside kitchen wall as in the picture, approx. 20 mm thick) is to be installed.
What should be considered to avoid problems with mold, etc., as much as possible? Space for an insulation layer is limited to max. 5 mm.
Can anything still be optimized at the transition from the window to the shelf?

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Albau from 1989 (uninsulated 30 cm brick wall)

Uninsulated is not quite correct, since for the construction year 1989 it is probably not solid bricks, but rather perforated bricks like Poroton or similar, as back then U-values of 0.5 W/m²K or lower were already required for exterior walls.

What should be considered to avoid as many problems as possible regarding mold etc.? Space for an insulation layer is minimal, max. 5 mm.

Use window blocks / shims for alignment, press them against the window, apply mounting adhesive and go for it. If there are cavities, or if the panel generally needs to be higher, level it with mortar beforehand. At the window stop you can also insert a soundproofing pad from the sanitary sector, similar to edge insulation strips but thinner. This way you avoid direct contact between the worktop and the window and simultaneously create a backing for the silicone joint.