Got it!!
Specifying Compiler Targets - Win32 apps | Microsoft Learn
Pixel shader for 9.1 and 9.2 (similar limits to ps_2_0)
I always thought ps_4_0_level_9_1 is some kind of fallback for XNA shader level 3 … falling back to level 2 and introducing the error makes it even stranger to me.
Anyway. ps4 works and I am out of here 