Roof rack and roof box

Hello. I have the following problem. I have an octavio combi rs 2.0 tsi mk3.5 and I installed a towbar and bought a bike rack. I have two e-bikes with a total weight of 40kg without batteries and a children's bike. So, all together with the rack, I am at the upper limit of the load capacity (75kg). In 3 days we are going to the seaside and we said that we would take the bikes with us. The trunk will be fully loaded in the back of course. Here I start to think whether the back will be too loaded and close to the ground? But we are also going on a ferry and I am afraid how it will go up. From all this, I thought of a roof box (I was looking at thule motion 3 xl and wingbar evo or edge crossbars) which would put pressure on the middle of the car, not the back. The car has a panorama that opens outwards. Does anyone have any experience regarding the weight of the back and regarding the crossbars and the rack, is it possible to open the roof window so that the glass does not hit the supports and break everything? Any help would be appreciated. Many thanks in advance.