I just re-read your post more carefully and realized I missed a big part.

I missed the part about you wanting to shift towards working less at the day job.

That conversation could be as simple as "starting on [some future date] my availability will be reduced to [x hours] per week. Let's figure out how to make that work with the least amount of disruption."

I've had similar conversations. In one case, the company wasn't happy, and I figured out a way to hire someone to do the other gigs so that I could stay 100% on the project.

That's also an easy way to start a business with actual employees. In my case, I hired two people, and the company was delighted because now they had a trusted partner, didn't have to do payroll, worry about benefits, or anything, and still get what they wanted (for cheaper). Don't underestimate how much a company might be willing to do to help make your goals a reality so that they can further their own.