Many times in the past I have combined the free version of Grammarly with the Hemmingway App (not an actual app, but a website so named). It isn't ideal, but does the job well enough.
If you're looking for a solid alternative you may consider
ProWritingAid. It's $79 per year and is actually a fair bit more in-depth than Grammarly in my experience. If you dig around youtube, there's almost always someone offering a discount code on top of that. They offer a free trial that I consider one of the best in the world. You get 30 days of use, but rather than continuous, it only counts for days you open the program. That lets you stretch the trial period out in very useful ways.
There is a downside however. If you get anxiety over 'errors', it may not be right for you. It offers up so much information that you can actually break your own writing if you aren't careful. Worrying too much over things like adjectives or how often you use the word pickle in a given text can make you lose sight of the prose itself. Something to be cautious about going in.