It is important to ensure a good air flow in the enclosure. Whether it's push, push-pull or just pull makes little difference to the radiator itself. I tested it with a 240 and a 360 radiator and all results were within 2°C at the cooling liquid. So practically zero difference at the GPU or CPU. Ultimately decided on push setups (3x120 + 2x120) for both radiators so you don't see the fans inside. The air is then transported out of the case with 1x140 + 3x120.

2 x 140mm radiators would also be possible. I was thinking of the ML140 Pro.
I would be super grateful for any suggestions!

Toutcas generally you want to blow the cool air through the radiator (i.e. from the outside). You can't install all fans in such a way that they blow air out of the case anyway, that makes no sense. So you have to install them over the radiator (to keep the cooling water as cold as possible) and usually blow them out of the back somewhere (depends on the case).
Besides, 2 140s move less air than 3 120s, so it would be smarter to have 3 120s and then throttle them down in speed (if you want it extremely quiet). Apart from that, you also cool the whole 360mm radiator and not just 280 mm of it.

As far as I know, it is always better to move the air out of the housing than to let it in.
However, I would also recommend the 140s, as more air can be moved with less rotation, which in turn is reflected in the noise level 1 to 1.

and if you do decide to go for a push/pull configuration, I would definitely prefer 6x 120 over 3x120 + 2x140.

It does make a difference, but it's so small that it's not worth it. With push/pull you will have a few degrees less than with push only. In addition, you also have to consider that the whole thing will be thicker than it already is. So for me personally it wouldn't be worth the money, but you have to decide for yourself.