I changed from "regular freelance" project work with 2 to 4 clients at a time, to contract work one client at a time, usually as an extension of their dev team. This has cut negotiating time significantly, I can bill pretty much 35-40 hours for the week and allows me to focus on coding, this suits my style of work well.

You can earn quite a bit if you're efficient, though this model is not that scalable beyond that e.g. it's difficult to build an agency / consultancy with full-time billing and very low negotiating overhead. Your role will turn more into that of a manager + salesperson as you hire more people.

Acquisition cost is very high. I spend most of my time selling myself than any other activity. I've been doing consulting since 2015. My network has grown to 5k, yet I still have to come up with new ways of expressing the same things.

I have the exact same problem. Feel like I spent 20-30% of my time chasing down new leads or going through the obligatory 2-3 rounds of intro/prospecting calls. Have you had any luck with building out lead pipelines?

Absolutely this