What's a word similar in usage to "diatribe," but not as harsh?
Solution 1:
Personally, I would use either "rant" if I am complaining about something but not attacking the recipient, or "rambling" if I'm just taking way too many words to say something. The point of a friendly letter is to be friendly. Ramblings tend to get skimmed over if the reader thinks it is not worth the effort to read so much text. If you are truly writing a friendly letter try to be concise while also being respectful of the reader. There really shouldn't be diatribes, rants, or ramblings in a friendly letter unless you know the other party very well.
Solution 2:
"Litany" or "polemic" would work.
Solution 3:
rant? screed? panegyric?
Solution 4:
"Harangue" might work.
In Merriam-Webster, "diatribe" is listed as a synonym and "lecture" as a definition, so it seems to be defined pretty broadly. I understand it to be more long-winded and emotional than a lecture, but not as strong as the OED's definition of "diatribe."
I'm not totally sure about the persuasive bit, though. Maybe "pitch" would work?
Solution 5:
A digression can be used if you are off-topic. A lecture if you are very thorough. A speech if it is well-rehearsed and given frequently. If I am passionate, I may get on my soap box.