Microsoft CRM Custom Service Layer

I have been asked by a client to create a custom app using an on premise MS CRM 2016. I would like to centralize my custom business logic in a service layer that can be accessed from plug ins and custom web pages via ajax. The client insists on keeping the app within CRM - I have proposed that we centralize this logic in a separate web service that lives on the same web server but they didn't like this option. I am not looking forward to embedding my business logic in little code turd plug ins and do not want to write business logic in Javascript.

Is it possible to create a service layer in CRM?
Is it possible to create a self-hosting website to be hosted within CRM? I know next to nothing about self-hosting apps...

Thanks in advance,

P.S. Bonus points if you can point me in the direction of any open sources for patterns on best practices for creating actual, complete SYSTEMS using MS CRM rather than the 9 million plug ins samples I've come across.