Hello, Zuzka! I am 100% certain this answer is correct: 'I have been considering writing to him for a long time, but I didn't have the courage to do so. ' Are you interested in improving your English? I am a professional English teacher, and English is my native language. I am currently accepting new students and would be happy to help you improve. Feel free to message me if you are interested in booking a lesson!

I've been considering writing him for a long time, but I didn't have the courage to do so.

In spoken casual English this sentence would be acceptable, but correct grammar would be “I have been considering writing TO him for a long time, but I didn’t have the courage to do so”.

All suggestions sound good to me. With the caveat that it should be "writing TO him".. next question you can ask is what does caveat mean and has John Used it correctly what is your opinion.

You could also structure the sentence this way. For a long time, I have been considering writing to him but haven't had the courage to do so. Or For some time, I have been thinking about writing to him but have not had the courage.