Answering your last question, a lot of people spend time writing the language or reading complex serious books. From personal experience, listening and reading simple English (summaries, an additional page of papers were experts focus on certain issues, human rights, justice, travel) helped me a lot first. The start translating your thoughts into words and use the spell check, once you're beyond this then grammar will often be correct expect for long sentences and first-person sentences. Listening helps in learning the language without even you being aware of it (try BBC or English documentaries). Learn similarities and new words and their association. Learning phrases rather than individual words help in faster improvement.
You can try languagetool.org or ginger software, both are for chrome. I would still suggest sticking to Grammarly as it does a good job and the website also quotes that Grammarly checks for plagiarism which would be an added bonus.