we are sorry to hear, you are having problems porting your plugins to Cinema 4D R20.
We are a bit surprised, you never contacted us for help. At least I'm not aware you did, neither via mail nor in this forum. Also I don't think you tried to apply for MAXON Registered Developer. Maybe with enough time your frustration reduces a bit and you are willing to accept our helping hand.
Seeing your plugins are quite old, maybe R12 or R13 the main chunk of work will be to follow the changes introduced with Cinema 4D R15. But believe me, it's not as bad as it may look, when running the compiler the first time. It's mainly find and replace work. See Transition to Cinema 4D R15 API and it's child pages in the SDK documentation.
For the remaining stuff and especially for setting up the project correctly, we have a Plugin Migration section in the docs.
Also here in this forum, Donovan has written a nice thread on this topic: Compiling the Cinema 4D R20 C++ SDK Examples.
To give you a rough idea, it took me roughly four hours to port a plugin of mine consisting of roughly 4500 lines of code written for R12 to Cinema 4D R20 (completely free of compiler warnings in the end). It depends of course on many factors and the complexity of a plugin, but I think, in many cases it's really not that bad. I also deliberately took the oldest version of the code I had, if I had taken a post-R15 version, I would have been done in less than an hour. And believe me, I'm not a fast coder.
In the end, I'd like to emphasize once more, we are really willing to help.