Really sorry if I critically misunderstand the scope of your question in OP (the balance of the primary source analysis and the status of research on methodology) since my brain is totally dead now and I'm not native in English.

Arnold, John. What is Medieval History? 2nd expanded ed. Cambridge: Polity, 2021 (1st ed. 2008): generally covers both trends (primary sources and research methodology plus trend of research), so I suppose this is the closest one OP is looking for.

On the other hand, Joel T. Rosenthal (ed.). Understanding Medieval Primary Sources: Using Historical Sources to Discover Medieval Europe. London: Routledge, 2012: serves the generally excellent overview as a collection of essays on various types of primary sources in Medieval Europe and possible approaches to analyze them.

Glenn, Jason, ed. The Middle Ages in Texts and Texture: Reflections on Medieval Sources. University of Toronto Press, 2011. http://www.jstor.org/stable/10.3138/j.ctt2tv21f. : is much more light touch collection (primarily as reading materials for undergraduate seminar, I suppose).

As for the theory and methodology on "auxiliary fields of research", the handy classic is probably still James Powell (ed.), Medieval Studies: An Introduction, 2nd ed., Ithaca, NY: Syracuse University Press, 1992, but its seriously dated (1st ed. was published in 1976). In French and in German, there must have been better alternatives even as a handy format, like Hans-Werner Goetz, Proseminar Geschichte: Mittelalter, 4th expanded ed., Stuttgart: Ulmer, 2014 (1st ed. 1993). Some individual field of studies like manuscript study also now have an independent research companion like Raymond Clemens & Timothy Graham (eds.), Introduction to Manuscript Studies, Cornell, NJ: Cornell UP, 2008..

If you are also looking also for the book on historiography as well as its related change of approaches on various individual topics in the Middle Ages, I can list some more introductions easily (so don't hesitate to specify).