You can use apps like https://aligntoday.com/ to help you manage clients, as to the best platform there is no one best. You can use upwork, clarity, fiver and a lot of self promotion. Go to networking events, pitch days, blog on medium, create youtube videos. It's a consistent combination that will help you build a sustainable career. Attract people with the knowledge you have.
Developers are key to providing intel on which platform is the best technical support. However, developing users - both coaches and clients is what will drive your revenue and freelance sites aren't the best partnerships. Let's talk about your revenue channels soon.