https://www.grammarly.com https://www.onelook.com https://www.wordreference.com + https://forum.wordreference.com/forums/english-only.6/ https://www.usingenglish.com http://conjugation.com/ Just some stuff I used in the past...
mmmm let me suggest you some more fun not orthodox alternatives: https://scattergoriesonline.net/ https://skribbl.io/ Both are guessing games of the fast pace where you will need to exercise your English vocabulary, they could be an option for when you are feeling tired and want to change of pace.
Just read more and write more. My vocab really expanded because I read many things and wrote a story while I'm at it. It really forced me to find the words that I wanted to express the correct meaning. It's also the same for grammar. Just read more or practice some exercises once a day. Also, just Grammarly to correct mistakes, because no one is perfect. PS: Grammarly Advanced Mistakes are bad and I recommended you to not listen to it. Most of them are just choices of preference.
As a native English speaker, the best methods (that I have found) to check grammar are: 1) To read it out loud. Most mistakes in grammar distinctly sound off or make you lose your breath. If it feels awkward to say, then it is probably an awkward sentence. 2) Simply identifying subject and predicate of a sentence. When I did some work as an editor during my time in university, the most common problem that I encountered was run on sentences. For example (This is not meant to be nit picky. I just wanted an example. And currently, I'm super lazy): "I've been steadily improving my grammar with Google-shifu but now that I wish to write a story, I want it to be fucking perfect so I need help to break through my current bottleneck." is not quite right. "I've been steadily improving my grammar with Google-shifu. But now that I wish to write a story, I want it to be fucking perfect. So, I need help to break through my current bottleneck." 3) Find someone to edit. Grammar mistakes are pretty much inevitable on initial drafts. They do start to disappear after a lot of practice, but it is always helpful to have a second set of eyes look over your work. 4) Grammar checkers/engines. This is hit or miss. There are definitely a lot of web sites out there that offer grammar engines. But in my experience, they, miss mistakes or mark correct grammar as being wrong. That's why if you decide to use them, use multiple and compare their results. If you can't figure out which is correct, ask for help from an expert (or google-shifu). I hope this helps! P.S. A helpful document the sums up proper comma usage: https://docs.google.com/document/d/179oQAEHSwiH7u78lwV0v658REny-T7zBTVAYiHhFOXQ/edit
Read online newspaper articles. Rather than learning the rules of grammar separately, it's better to get a lot of input (read a lot) and familiarize yourself with common structures used in English language. Since you're aiming to write a story, find a book/story that you like (not WN LN materials, please... published books, because those have gone through professional proofreading and editing) and try to imitate that writing style. In writing stories, stylistic choices often trump core English grammar. For example, you don't start sentences with conjunctions (such as AND, OR, BUT). Sometimes you write incomplete sentences to show tension or create a certain mood. It's good to learn and understand English grammar, but if you can get your point across without jarring errors it's fine. Stick to basic rules, spelling, and punctuation and you'll be fine. Read and imitate excerpts from books and especially newspaper articles.
Seconding ebonykun's suggestion. You already know enough to communicate, now the best way to improve is to read read read, but this time professionally edited material.