As already stated, artistic skill is the main factor together with other tips you already got.
However, there are some things on the tool side of thing that can help.
I'm not very good at likeness myself but the best result I have reached doing the following.
Going digital.
Being able to zoom in, using reference images more creative and having access to an undo helps a lot in my opinion. Now days I mostly 3D print but now and then I want to do something in Sculpy and when I do I 3D print a negative and push the clay into it to transfer the face into the clay. Sculpting faces directly in Sculpey have never worked for me in that scale. But hey, I might just miss the artistic skill for it
If I for some reason not could sculpt digital and 3D print, then I would go back to the way I would do it before the time of the 3D printers and use a hard toy/sculpting wax.
My favorite is Toxic Papas wax that I bought probably 20 years ago or so
Don't think that one can be bought any more.
Working in Sculpey is in my opinion hard as the material is rubbery, you can push it and it bounces back a little, even the harder grades, if you push a little bit to hard you deform it etc.
A hard wax is hard as, hmm what to compare to, well a hard wax.
With a metal tool you can shave of material and to remove more material at once you have to heat the tool.
To add material you have to melt the wax and drip it on (unless it's a hybrid with also have clay properties). This gives excellent control when getting the small details in place. You can polish it to mirror finish using a nylon stocking.
A drawback of using this material is that it is not a final material. You have to make a mold and cast the head or maybe just make o mold for the face and press sculpey into it to transfer it to that material.
An fantastic and fun material to work with in my opinion. And also the old school pre digital way of making masters for action figures.
Just to get to some starting point to look in to if it could be something for you you can check out CX5 wax that's a hybrid between a pure hard tooling wax and clay. If heated it gets clay like and when cooled down it's hard. Not as hard as the wax I used but that did not have the clay state option and did go directly from hard to liquid when heated. It has it's pros and cons.
https://www.youtube.com/watch?v=8zYkiEOrnKs
And yeah, the material is reusable so once you made you mold you can reuse the wax for something else.