It sounds like these classes are fairly common to much of your project. You might try some of these:
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Do your best to break apart
ProjectForwards.hinto several files as you suggested. Make sure each subsystem only gets the declarations it truly needs. If nothing else, that process will force you to think about the coupling between your subsystems and you might find ways to reduce it. These are all good steps toward avoiding over-compilation. -
Mimic
<iosfwd>. Have each common class or module provide its own forward-include header that just provides the class names and any convenience typedefs. Then you can #include that everywhere. Yes, you’ll repeat the list a lot, but think about it this way: nobody complains about #including<vector>,<string>, and<map>in six different places in their code. -
Use Pimpl more often. This will have a similar effect to my previous suggestion but will require more work on your part. If your interfaces are stable, then you can safely provide the typedefs in those headers and #include them directly.