Studies exploring complexity, accuracy, and fluency (CAF) development from a complex dynamic or dynamic usage-based (DUB) systems view are well documented in the literature (Verspoor et al., 2021). However, the analyses in these studies are mostly limited to elementary to intermediate level L2 learners from instructed secondary school settings. The current study thus investigates the relationship between time and holistic judgments of proficiency and syntactic and lexical complexity development in the writings of a group of advanced sojourners during a semester spent abroad, addressing the empirical gaps pertained to different proficiency levels and context. The data are weekly diary entries produced over the course of a 12 to 17 week stay abroad. The data are analyzed for holistic proficiency gains and syntactic and lexical complexity measures through linear mixed-effects modelling. The results confirm the significance of random individual variation, but also suggests a common developmental path as the participants ended up with significant gains in their holistic proficiency but with more elaborate sentences with fewer verb phrases and less diverse vocabulary.