I’ve noticed that many Greek letters have the same pronunciation: (υ,η,ι, ει, οι) and also (ε, αι), and possibly others.
Even though I can pronounce Greek words easily, I am having great difficulty writing them correctly.
What is the best way to learn Greek spelling? Any tricks, hacks or shortcuts?
I think there is no shortcut. There are certain patterns which you will certainly discover yourself during your studies. Among the activities on lingq, you could try dictation.