Have you heard of the book "MSP in a Month"? It might be worth a look at the author's Karl Palachuk's other books about service agreements and reselling services. I have not read them but have considered it a plan "B". Plan "A" is goat farming.
I just looked around but didn't find this exact title. Is it "Managed Services in a Month"?
https://www.amazon.com/Managed-Services-Month-Successful-Consulting-ebook/dp/B078J7ZKV1