You clearly state that you understand all that your friend has said, but the description of your level of agreement with the content of the messages is "somewhat" less clear. It's, therefore, not totally clear to me if your hesitation to respond comes, at least partially and/or subconciously, from your lack of total agreement with the messages.
Since you don’t fully agree with (the gist of) the messages you could respond hedgingly with:
I’ll just say that it looks like you’ve covered/touched all the
bases and I
have nothing more to add [at this time].
If you do agree with at least the important points mentioned by your friend, you could hedge a bit less with:
I think you’ve covered/touched all the bases quite nicely and there’s
nothing more to add.”
"cover all the bases (American & Australian) also touch all the bases (American):
to deal with every part of a situation or activity"
(from Cambridge Dictionary of American Idioms via ‘The Free Dictionary by Farlex’)
(Granted, these responses are perhaps too polite, 'cause they'd probably be interpreted more as you complementing him on his thorough coverage of the subject than as you revealing your lack of interest/knowledge in/of the subject, but if you don't mind going full polite, they might work)
For an expression that relates solely to your silence without giving your friend the pleasure of thinking that he has “covered all the bases” (quite nicely or not) with his flurry of messages, you could simply:
Take/plead the Fifth
I take the Fifth
I take/plead the Fifth (Amendment)(American
humorous): something that you say in order to tell someone you are not
going to answer a question
Usage notes: The Fifth Amendment is the
part of American law that says someone does not have to answer
questions about themselves in a law court.
(again, from Cambridge Dictionary of American Idioms via ‘The Free Dictionary by Farlex’)
I’ll take/plead the Fifth [on that] on the grounds that anything I say
may jeopardize our friendship!