The article by Georg G. Iggers (1926-2017) is heavily saturated with relevant information on the topic of historicism. I recommend it for anyone interested in the development of historical studies. It was well-written and included topics for future research at the end.
He was a renowned historian of historiography.[3, p.335]
Here are some of the books he authored:
The Cult of Authority (1970)
Historiography in the Twentieth Century (2012)
The German Conception of History (2012)
A Global History of Historiography (2017)
Iggers began by giving an overview of the three schools of historicism which exist simultaneously with but independently from each other. In my own designations, they are relativism, 19th & early 20th century historiography, and neo-historicism. His present article only deals with the first two schools.
He gives a footnote on what to read pertaining to the history of the term historicism. Among the plethora of sources provided, he first named Lee & Beck (1954), which I have taken notes on and published here.
The most important representative for historicism in 20th century Italy was Benedetto Croce. Croce, Gasset, Collingwood, and Meinecke all believed that naturalism was insufficient “… to understand human reality because of the uniqueness and individuality of the historical world.”[1, p.135-136]
Muhlack and Meinecke saw historicism as the zenith of historical understanding.[1, p.145]
There’s a bit in this article about history becoming a science or a profession. It repeatedly says that this occurred around 1800.[1, p.149]
Key figures: Friedrich Schlegel, Novalis, Ludwig Feuerbach, Christoph J. Braniss, I. H. Fichte, Carl Prantl, Karl Werner, Giambattista Vico, Benedetto Croce, R. G. Collingwood, Hegel, Leopold Ranke, Hegel, Droysen, Wilhelm Dilthey, Wilhelm Windelband, Heinrich Rickert, Eugen Duhring, Carl Menger, Adolf Wagner, Wilhelm Roscher, Karl Knies, Gustav Schmoller, Friedrich Meinecke, Ernst Troeltsch, Max Weber, Friedrich Nietzsche, Albrecht Ritschl, Karl Barth, Rudolf Bultmann, Friedrich Gogarten, Reinhold Niebuhr, Martin Heidegger, Otto Hintze, Karl Heussi, Jose Ortega y Gasset, Giovanni Gentile, Antonio Gramsci, Giuseppe Galasso, Fulvio Tessitore, Pietro Rossi, Giuseppe Cacciatore, Wilhelm von Humboldt, Ernst Cassirer, Karl Popper, Otto Gerhard Oexle, Annette Wittkau, Charles, R. Bambach, Wolfgang Hardtwig, Jorn Rusen, Horst-Walter Blanke Schweers, Friedrich Jaeger, Dirk Fleischer, Hans-Jurgen Pandel, Ulrich Muhlack, Jeremy Telman, Thomas Kuhn, Charles R. Bambach, Georg von Below, Eckart Kehr, Hans-Ulrich Wehler, Wolfgang Mommsen, Jurgen Kocka, Georg Iggers, Thomas Nipperday
1797: Friedrich Schlegel wrote down the term Historismus, which is now the earliest known use of the word.[1, p.130]
I think the usage of it was to note the differences between the times between when he was writing and what he considered antiquity.
1798: Novalis mentioned Historismus but didn’t offer anything to let us know the definition.[1, p.130]
c.1800-1866: Ludwig Feuerbach, Christoph J. Braniss, I. H. Fichte, Carl Prantl, etc. used Historismus in similar ways to Schlegel.[1, p.130]
1922: “The Problem of Historicism” was put forth by Ernst Troeltsch.[1, p.133]
1932: Karl Heussi, Die Krisis des Historismus[1, p.135]
1936: Friedrich Meinecke, Die Entstehung des Historismus[1, p.135]
1940s: In the English speaking world, the word historicism began to be used in place of historism.[1, p.137]
1975: Ernst Cassirer, The German Enlightenment and the Rise of Historicism[1, p.150]
1989: Veeser, The New Historicism[1, p.137]
“Karl Werner, in his 1879 book on Giambattista Vico, saw the core of the historicist outlook in Vico’s notion that the human mind knows no other reality than history: history is made by human beings and therefore reflects human intentions, that is, meaning. Nature, because it is not made by humans, reflects no meanings which can be understood in this way.”
Igger (1995)[1, p.130]
“One might have hoped that critical study would unmask myths which had instrumentalized history in the service of political and social ideologies, but the opposite was generally the case. Historical study reinforced historical myths.”
Igger (1995)[1, p.139]
 – Iggers, Georg G. “Historicism: The History and Meaning of the Term.” Journal of the History of Ideas, vol. 56, no. 1, 1995, pp. 129–152. JSTOR, www.jstor.org/stable/2710011. Accessed 26 Mar. 2021.
 – Daum, A. W. (2018). Georg G. Iggers (1926–2017). Central European History, 51(03), 335–353. doi:10.1017/s0008938918000626. https://www.cambridge.org/core/journals/central-european-history/article/abs/georg-g-iggers-19262017/12522F32F1F6D1EE9A8E2C1B3015E9EC. Accessed 4 Apr. 2021.
 – https://www.worldcat.org/wcpa/search?q=Iggers+G.+G.&qt=owc_search. Accessed 4 Apr. 2021.