Practice writing. Just write whenever you can, in whatever environment, no matter how unformal. Your company wiki, your personal blog, whatever.
Maturing from a "junior" engineer into a more experienced one usually means initiating suggestions, not just following them. As the OP said -- have an opinion! But your suggestions will always be limited to how well you can communicate them. And a lot of your interactions wherever you work will be with non-technical people, so the better you can frame technical situations to them, the more they will enjoy working with you and cede to your judgment.
I've seen so many talented software engineers not find the professional success they deserve because they couldn't write. You will do brilliant things, and those brilliant things will make your employer money, and your ability to write will matter a lot in how much that brilliance is valued. Likewise you will make mistakes, and sometimes those mistakes will cost your employer money, and when that happens someone will be upset, and your ability to write will matter a lot in how soon they calm down.
Some of the most important keystrokes in my career were not for writing lines of code, but for how I composed an email. I'm not saying your expertise in software development doesn't matter -- of course it does, and a bad software engineer that's a good writer does not make them a good software engineer. But your ability to write and communicate can be a tremendous force multiplier for your career.