The current two studies examined the effects of an instructional unit on synthesis writing for 9th grade students on holistic text quality. Because students' writing routines have been shown to affect the effectiveness of writing instruction, we designed a unit that aimed to be equally effective for all writers, regardless of their routines. On two occasions in the unit, we provided students with options to choose between a Preplanning or Drafting strategy. They could create an individual learning path within the unit. We tested the effects of the unit in two consecutive studies with independent samples, using a quasi-experimental pretest-posttest design with switching panels. Study 1 included three teachers and 152 9th grade students from five classes in one school. We found a significant effect of the unit on text quality in both panels. However, analyses showed that this effect was moderated by writing routine. To generalize the effect across writing routines, we provided students in Study 2 with an extra session on metacognitive knowledge about (synthesis) writing processes, enabling them to make better-informed strategy choices. Study 2 included six teachers and 233 students from 10 classes in three schools, all different from Study 1. Analyses again showed a significant effect of the experimental condition in panel 1, which was replicated in panel 2. Moreover, the effect was no longer moderated by writing routine. This indicates that the redesigned intervention was equally effective for students with different routines.