I've just changed it to highway=secondary.

In my opinion, this is the best balance between the arguments presented here. By no means definitive, if you disagree of have further arguments, feel free to re-open this note and discuss.

Summary of my reasoning:
- Njegoševa and the rest of Šmartinska are highway=primary.
- However, as pointed out by lukap1 and StefanB, this section's significance is somewhat reduced by hgv=no and oneway=yes.

I think highway=secondary is an appropriate compromise (demoting it by one level with respect to Njegoševa and the rest of Šmartinska).
Also, this is the same highway type of other roads with similar significance to the road network, which also have restrictions (Prešernova, Dalmatinova).