I am not sure if this is the best site for what you seek then. Most of the people on this site started small (either sport bike or cruiser) with a few folks in the 650 range and even fewer on larger bikes. Almost to a man (and woman) we are all beginners though. I think there are maybe 3 or 4 folks on here with many years and bikes behind them. Most of us will not be able to your questions.
Having said that, if you trawl through the posts I know there were threads that talk about the pros/cons of inline 4 engines, V-twins and single cylinder engines. I know not all 600cc bikes are the same but I cannot tell you what makes one good and one bad.
I can tell you there was some advice I ignored before buying that I ignored that I now know to be true.
Your first bike will not be your last
This is oh so true. I have only had my bike 4 months and I love it but I am still drooling over and planning what my next purchase will be. It’s like a crazy addiction.
Based on the first point, this one makes more sense. If you accept you will be buying multiple bikes then trying to find one bike that fits all your needs becomes irrelevant. Give yourself over to the fact you will be starting an expensive bike habit 
