How to connect IKEA cabinet and tabletop with connector?

That was also my first thought, perforated sheet metal but there are a lot of cavities in the cabinet.

At the very edge, the Kallax might work. But then you have a height difference between the Kallax and the tabletop.

That can be compensated with additional feet or pads. Only I don't know if a perforated sheet holds up well, as there is already quite a bit on the table and I also want to work on it.

In furniture construction, you can also make such planar connections with wooden dowels, etc. So drill into the connecting surfaces and insert wooden dowels. Due to the materials here, these would probably need to be dimensioned larger and drilled deeper. In addition, very precise work is necessary, otherwise the board will be connected crookedly or there will be a gap to the shelf.