Isn’t this default behavior? The power should go down when switching from TO to CLB as TO is almost max power which puts strain on the engines, while CLB is more economical. As a result the nose drops down, and if done too early (below 1000ft thereabouts) you’ll get some “don’t sink” notifications while it levels out until it reaches 170kts. Then it starts climbing ever so slowly. Waiting until you’re around 170kts will work I suppose.
Once it’s on Cruise it’ll set the Cruise speed. Since the latest 1.0.23/34 patch the Cruise speed it sets is about what it will reach. Before that it would set 245 and never reach it, which is fine as it means drag equals power.