Hello there Tom,
Thank you so much for your swift reply.
It was indeed the 5px padding in Elementor which you’ve mentioned. For some reason Elementor applied 5px padding in the column of the main menu, even though it wasn’t visible. Clicking the padding-link-button in Elementor (unlocking it) fixed it (see screenshot: https://ibb.co/K6phtRf)
The final result is definitely as requested.
Thank you very much for your insights, you helped me out greatly with this one!
Wish you all the best!