Compiling forms in C#

Basically, you are working with solution. It can contain multiple projects. For each dll, you must have one project. So create 80 projects, add to each of them single form, edit it, add some logic.

Then there will be a main project, which produce exe. You can reference all dlls in that project, but better don't. If you do, updating any of dll will required recompiling that exe too. You can load them dynamically or use sort of plugin system (to enumerate dlls, understand their purpose, etc). Then you obtain Type from assembly (loaded dll), create instance (which will call constructor, which calls InitializeComponents, which loads form resources) and display form.

Regarding abstraction, you surely need something. To example, login window. You can create a generic form with some focus, user interface and user interaction logic. But it has to communicated with main project (which encapsulate encryption, password storage model, user rights, etc). One easy way to do this is to provide 2 interfaces:

interface ILoginImplementation
{
    public void SetInitialUserName(string name);
}

interface ILoginLogic
{
    public bool TryAuthenticate(string name, string password);
}

Implementation is what your form must implement and Logic is what main project implements and supply when instantiating login form.

I realize this is probably not ideal, but I still think your best bet is to use Visual Studio and create a separate project for each .dll to be created.

By right clicking the Solution node and selecting Add > New Solution Folder, you can at least organize your projects into a somewhat more orderly hierarchy. That alone might go a long way to make your project more manageable.

PS: If you haven't already, you should definitely try to create an interface, or a base class (or both!) that each of your Form-classes can derive from or implement. If you're able to abstract away and generalize some of the logic, it is quite likely to save you a lot of work down the road.