A lengthy piece has recently been published on the authorship of the Pastoral Epistles:

Schmidt, Karl Matthias. “Drei aus zweiter Hand: Die Pastoralbriefe und ihre Autoren.” Studien zum Neuen Testament and seiner Umwelt A 46 (2021): 71–151.

Abstract: “Within a discussion on the literary form of the Pastoral Epistles becoming more complex this essay gets in line with those interpretations, which assume three pseudepigrapha, written by three different authors. Form and contents suggest that the letter to Titus is based on the Second Letter to Timothy and that the First Letter to Timothy is depending on both predecessors.”