is there a debug functionality outside a fully fleshed-out IDE?

Firstly, Visual Studio isn't the only IDE in existence. Secondly, for most code editors you can set up some sort of a debugger interface.

of course I can't debug entirely in the terminal

You can. I highly recommend familiarizing yourself with gdb - the OG command-line debugger. I also suggest you check out cgdb - a TUI (text user interface) debugger based on gdb. Useful stuff and easy to set up!

Is it viable to code outside an encapsulating IDE such as Visual Studio, with standalone replacements for the features that an IDE provides?

This is something that everyone discovers for themselves. I suggest you try out both - creating your own "IDE" from replacements and using an actual IDE. See what you like more.

But, on the topic of Visual Studio, what I want to recommend against is fully relying on Visual Studio's solutions as your main method of building applications, because that limits you to Windows apps. If you are planning on building closs-platform apps, I suggest you also learn CMake in case you haven't already. CMake can generate both Visual Studio solutions and also Makefiles (that you use to build stuff on UN*X systems, for example). Might be off-topic, but I just thought I'd throw it out there