Why not create 2 projects?
one with full featured ServiceStack to create the API, and then other in .Net Core for only the frontend, where you use the
ServiceStack.Client.Core to actually consume the API made in the first project. (you can even keep both in the same solution in Visual Studio)
Simply host the API in a host and the frontend in other (api.domain.com + domain.com)
This way you have the best of the 2 worlds… and, enabling the new OpenAPI feature makes it even easier to consume with other platforms, for example, Azure API Management.
This is how we do everything around my current company, we have several frontends that consume microservices (small ServiceStack API’s) making it easier to maintain and scale each service.
And we have now converted an old frontend from WebForms 2.0 to ReactJs using .NET Core and consuming the exact same SS API…