You'll probably naturally gravitate to a niche whether it's projects in a framework you are familiar with or 'developing and scaling marketplaces' that you ment

You'll probably naturally gravitate to a niche whether it's projects in a framework you are familiar with or 'developing and scaling marketplaces' that you mentioned. You'll tend to move toward doing what projects pay the best and come find you. I would create a personal website and start blogging about the technical aspect and the scaling you do. I know most people say make one site that is your personal brand. But I always feel like you would create two sites a technical expertise site and a scaling site. But it's probably better to create a personal site and start posting regularly there. This could lead to technical projects and scaling projects from people finding your articles. Start building up an email list from the start, maybe one for technical/framework tips/articles and one for scaling. Once you start building up a reputation as an expert in each of these, engagements will find you and you can market ebooks and courses in each to your list. Being a freelancer/consultant I would recommend creating products/SaaS that generate recurring revenue so you are not so dependent on finding new project work. Make it part of your long term plan.