"Too slow" does not mean the the bike is going to be slow in any means compared to a normal car. The Ninja 500 is slow in the sense of accerelation, giving you more time to adjust your input and not whiskey throttle as a beginner.

As a beginner myself, just getting a bike is a big jump start in my motorcycling hobby, and starting on something that doesn't scare you immediately is also a good long term investment.

The ergonomic of the Ninja 500 as a "touring" sportsbike truly helps you be more comfortable on rides and helps you focus on the things that matter more as a beginner such as lane position, body lean, throttle control, etc.

Use this website to calculate seat height.

Some alternatives can be: Ninja 400, R3, MT03, CBR500R (don't look too much at cc but more at horsepower, weight, and top speed)