Have you heard of Reddit? There is a subreddit called r/FanFiction. There is a weekly thread where people offer exchanges of help. Usually it is not a 1:1 offer as it is usually an ongoing relationship, but the 'gift economy' means you give more help than you receive, usually based on word count.
Have you heard of FanFiction? It gets a bad rap, but there are writers of all levels of skill, from professional writers to school children who have this as a hobby. You can specify what kind of help you want, and what kind of help you need.
You can say something like:
"I have an original fiction work which is 100,000 words long. But you could help with part or all of the story. The story is complete, but I need help with grammar and punctuation. For example, a lot of my sentences are things like "Go home now." Sam said. (When the correct way of writing it is "Go home now," Sam said).
I can help people whose English is a second language proof read their story (but I would not be any help with the grammar). I can also help with Britpick, brainstorming or checking how well the story reads in general and I will offer feedback and suggestions."
You specify how long you want it to take, e.g. can they do it within one month or something like that (bear in mind, money is not allowed to be exchanged). The policy is you help for generally the amount of time it took people to help you.
You say what you don't want to read, e.g. "Stories rated teen and up only please, no sex, no horror or violence.
I can do either a 1:1 exchange for a story the same length, or if your story is shorter, a partial exchange. Over 18s only, please."
You create a gmail address, so that they don't have your real name and then paste your work into Google docs, usually each about 10,000 words long, for ease of scrolling. Depending on your book it will be about 5/10 docs long.
Don't be put off by an exchange with someone whose English is not a first language, often their grammar is perfect, but it's just their word choice which is sometimes off in their own story. Britpick is when someone (Usually American) writes something set in England, and you highlight and suggest changes for things like 'He got mad/sidewalk/he walked a block/he put on pants/he had a stack of pancakes for breakfast, etc. Things they didn't realise are not British.
Someone might offer to do it for nothing in exchange (because someone has helped them in the past, or just to keep up the spirit of support and helpfulness- gift economy).
Just like in real life, there will be chancers who only want to take without giving, flaky people who accept your help then don't fulfill their end of the bargain. Selfish schoolchildren who take your help but do a terrible job on yours. Sometimes it takes a few tries, but mostly people are lovely and passionate about their hobby.
I'm on mobile and I don't have pm, but if you're interested, or want to know more, reply here.