The reason for pop ups is that you had set it to "ask", so Avast was doing what you said it has to do.
If you don't want pop ups, you have 3 options.
A) leave automatic updates, and disable the pop ups (both have their relevant setting under "settings -> update"). Note: Do NOT confuse with the "settings -> popups" options, which is for something else.
B) instead of "ask" or "automatic", you can set it to "manual", so no "update pop ups".
C) "Silent mode". Note that there are 2 separate options with slightly different consequences. Silent mode is independent of the "update" mode ("auto", "ask", "manual").
Setting the automatic update mode and setting the "checking" periods to "many years" doesn't seem to be a logic move. I mean, you still want to be protected, correct?
Remember that if you chose the "ask" or "manual" update modes, you still need to manually update from time to time, even if you don't see the tray notifications about it.