In my main side project what I've done is separate different conceptual components in clojure libraries that I then import into my main program (clojure enables you to seamlessly require a private github repo as a library).
This way you get most of the benefits of separating a codebase (such as testing different things, leaving code that barely works in the repo, pointing to older versions of part of the monolith, smaller repos, etc.) whilst integration between modules is a one-liner, so I don´t need microservices, servers, information transformation, etc.
There's even the possibility to dynamically add library requirements with add-lib, an experimental tools.deps functionality.
This is an excellent approach that I wish was more well known/common. We use this first before going to a complete service, which becomes easier anyway once you have a few libraries that had been in use and tested by then. Works great for our clojure code and other languages.
This way you get most of the benefits of separating a codebase (such as testing different things, leaving code that barely works in the repo, pointing to older versions of part of the monolith, smaller repos, etc.) whilst integration between modules is a one-liner, so I don´t need microservices, servers, information transformation, etc.
There's even the possibility to dynamically add library requirements with add-lib, an experimental tools.deps functionality.