What you describe is an increasingly common model referred to usually as a Venture Builder or Startup Studio.
A venture builder enables startups to leverage its internal team of business developers, designers, developers, marketers and sometimes VCs in the ideation, development and launch of a company. It is a business model structured for acceleration: invest in a fully equipped, talented team, identify a few things you do well, replicate the model and put your team to work to grow different businesses in parallel.
The equity component brings alignment of business interests similarly to an in-house team but there is a significant reduction in risk. Hiring a CTO and development team is costly in cash and equity and there's little to guarantee success. Unless you have a big wad of cash in the bank this is not likely an option. The alternative then is an agency or a venture builder both bringing an on-demand team which is experienced and comfortable working together. They will have processes and workflows in place to ensure efficiency and can offer fixed price MVPs.
The differences between tech agencies and venture builders is significant:
Agencies VS Venture Builders