of a character than Beaver?

Like it made more sense to give Wally more screen time?

I just did a count from season 3 thru 6. I counted Wally (and his friends) - centered episodes. Some episodes were hard to figure out, such as "Substitute Father," which really involved both; I counted that one as a Wally episode, by the title, and the fact that the script was really centered on him. Others, like "The Yard Birds" involved both of them.

There is a definite increase in stories which are centered on Wally and his group:

Season 3 has 8 Wally episodes, season 4 has 10, season 5 has 13, and season 6 has 16.

As far as making sense, yes, I think. In the Wally universe they had a great actor cast as a great character: Ken Osmond/Eddie Haskell. They also reframed the Lumpy character, taking him from a threatening bully to a friendly big guy. Unlike in the Beaver world where all the scripts were centered on him, in Wally's world some scripts were centered on Eddie or Lumpy.

Also the cute, naive Beaver eventually grew up, narrowing the story possibilities. So, having three good teen actors and characters to work with, it made sense to use them more in the later seasons.