Although different empirical studies have recently explored the L2 writing–SLA interfaces, how L2 writers modify and consolidate their linguistic knowledge with different levels of perceived complexity remains underexplored. Featuring the qualitative phase of a longitudinal, mixed-methods study with the aim of complementarity, the current study explored the perspectives of 20 low- to mid-intermediate L2 writers about how their consistent engagement in an iterative process of writing, feedback processing and rewriting contributed to L2 development. In-depth explanations and numerous script-specific exemplifications in semi-structured interviews showcased the language learning potential of acting on direct WCF on an extended number of grammatical features over time, resulting in deep WCF processing and substantial cognitive and affective engagement with feedback. Results suggested participants' reliance on the prolonged process to (re)formulate and confirm hypotheses and increasingly accumulate category-based WCF instances from previous and subsequent compositions to abstract most of the underlying rules embedded in direct WCF. The participants explained how different challenges against rule abstraction were resolved, and why some remained unresolved. The study proposes a model illustrating a synergistic interaction between writing, feedback processing and rewriting for L2 writers to gradually consolidate linguistic knowledge. Theoretical, methodological and pedagogical implications of the study are discussed.