An IDE is pretty helpful for C++ as a project grows. Code navigation helps a bunch, the integrated debugger is far more friendly than raw gdb, and not having to juggle windows is convenient.
As for IDEs, feel free to give several a shot. I highly recommend trying out QtCrearor even if you aren’t using Qt at all. It is in my opinion the best C++ ide, and the minimal UI keeps distractions to a minimum.
Also, you mention using make files. You should really take a look at adopting a build tool such as cmake (the most common, but kind of a mess) or meson (which is much friendlier), as hand crafted make files don’t scale particularly well.
There is no legal requirement that your program output what license it is under as a part of any particular command, or that each file contains a license header. Providing the license once at the top level and indicating that the entire repository is under that license is sufficient to put all of the code under the license of your choosing. Anything more is simply to make it more obvious when looking at a particular piece in isolation.