[aleph_minus_one]

> Layer lines must fade away into invisibility.

The layer lines are much less pronounced when you use a 0.25 mm nozzle with an appropriate layer height instead of a 0.4 mm nozzle (the possible quality is even on the brink to satisfy people who use 3D printing for producing miniatures). The prize you need to pay is of course the print time.

> Top surfaces must be impeccably smooth without any stepping.

In the last years there was a lot of progress on ironing features in slicers, which mitigates this issue:

> https://help.prusa3d.com/article/ironing_177488

Another very recent addition to mitigate the perceived problem is the recent addition of "fuzzy skin" features in slicers, which by making the surfaces look "more rough" hides the imperfections of the FDM printing process.

--

Another solution is to simply use resin printing instead of FDM printing for finished products if feasible.