Recorders don't get uniformly trickier as you go higher up.

A lot of tunes derived from the fiddle repertoire go up to the high B (one where a recorder produces a much better result than a whistle is "The Old Grey Cat" - whistles will usually play the high B out of tune or with a harsh tone). The high B is easier to strike than the A below it on most recorders.

High C and D are a bit more specialized, and I there are two tunes I play that use the high E flat beyond that. The high D has an unexpected fingering but will sound easily on any recorder that can produce the A below it.

You are in for a truly interesting time with tunes that want a high C#. This is usually doable but there is no one fingering that always works. The Josephine Waltz (in A) is one tune that needs it. If I know I'm going to be doing that one I'll usually try to have a transverse flute handy.