Is there any other reason to position the PSU correctly other than temperatures?

I mean, temperatures are certainly an important consideration.

only low loads, meaning integrated GPU, SSDs and T-line CPU (power optimized)

You should be fine as long as it's not totally stagnant (which isn't hard to achieve). I have a somewhat similar case structure with a non-passive PSU.

I've got a Kolink Satellite Plus with a filtered rear intake (Silverstone FHP141), with an M9 Plus CPU Cooler (airflow still back to front), followed by my PSU. I'm currently using a Seasonic SGX-650, which is a semi-passive unit. I can feel enough air exiting the unit (to the side), even though its fan isn't on.

If you can set things up in your case so you have a basic airstream going through the PSU, it'll be fine, even without the ideal orientation.

Passive PSUs should only be used in a case where there is sufficient airflow. So long as air is getting to the PSU to allow for proper passive cooling, it should be fine.