Suppose I have a number of IHostedService implementations with a ton of dependencies that no WebApi controllers do not have direct nor indirect dependencies, too.
Ideally, I like it if could set up an IoC container just for the IHostedService (Isolated microservices using containers is not an option for me).
One option is that I could rip out the internals from https://github.com/dotnet/extensions/blob/494e2c53cd/src/Hosting/Hosting/src/Internal/Host.cs and start up my own non-web-related host, but if I could avoid it if someone with experience could please enlighten me
The asp.net core web host is built on top of the generic host.
You can separate how you configure your core services from your web specific services. You could also use command line arguments to decide what services to configure, within the same binary.
Though I would recommend moving your services to another library, writing helper functions to register all their dependencies, then building multiple executables for hosting web / backend services.