The ASP.NET Core Renderer does not limit you to working with ASP.NET Core pages only. Using the Renderer, you can simultaneously work with ASP.NET Core and MVC pages and maintain websites with a mixed collection of rendering solutions. This way, you can migrate your current website page by page.
For more information, see Migrate Pages to ASP.NET Core. You’re welcome to also review the complete Sitefinity ASP.NET Core Renderer product documentation.