I think the ossified errors acquire during immersion learning (Phase 3) can be (largely) avoided by pushing your comprehension with a dictionary and daily tutoring or formalized self-study. That way your understanding progresses along with your comprehension and use, and you don’t end up inventing grammar to make do. It also helps to have started off with a foundation via some Phase 1 (academic study) +/- Phase 2 (media consumption).

Have you traveled much to learn Spanish, or have you had good success with the Phase 2 media consumption approach?