Wait, why do you think "it's just not possible to scale that past 300k/yr"? I have at least one counter-example ;-)

In general, the way it's worked for me is if I have multiple potential new gigs, I ask for more money. When the next gig comes along, that's my new baseline. Over the last 6 years that's helped moved the hourly rate to a number I wouldn't have asked for upfront. I hate to say no to customers too, so sometimes end up working 60-70 hour weeks in bursts. It's worth it though, because when things get quiet, I can go snowboarding for a week without asking a boss for permission...