FSA crank + BB combination too tight
Ok, so I got this sorted!
Thanks to everyone for input. Posting the solution here in case someone else is facing the same problem.
The answer is inspired by @Nathan Knutson 's post.
@Nathan mentioned a transparent scuff washer... And I did not remember seeing one when taking the crankset off. So I took it all apart again and specifically looked for it. There is indeed a transparent plastic (or nylon?) washer that was tightly pressed into a small groove on the inner side of the NDS crank arm. It was flush with the surface and did not come off when I was cleaning the crank so I assumed it is part of the crank arm and not removable.
Now I removed that washer and it freed about 0.3mm for the wave washer to expand. Asessembled, torqued to recommended setting, crank spinning well and I can clearly see that the wave washer is no longer flat - it now serves its purpose and preloads the bearing as intended.
There is a slight concern that removing the scuff washer will make the wave washer rub against the crank arm and scuff it over time but for me this is a minor concern as this crankset is cheap and I was close to replacing it with Shimano or SRAM dub or anything else with adjustable preload... Now it will serve a little bit longer before being discarded.
For all those who assumed misalignment of the BB shell - this is a valid assumption but wasn't my case.