Thanks @TLATER, for these explanations. I had earlier tried enabling Nvidia modesetting (which didn’t work), and, surprisingly, my Dell BIOS has no switch for disabling the Intel graphics unit.
I always thought Optimus mode is only for laptops so I hadn’t tried it, and now I’m happy with the current setup of blacklisting i915.
I guess you have answered my main question. I’m still curious if this should be documented somewhere - I didn’t think my hardware configuration is uncommon. Or maybe it would make sense if NixOS had a configuration option that ensures only the discrete GPU is activated.