But for my trading position I don't want to put the coins on a hardware wallet because that would be too cumbersome but I want to distribute the coins on several exchanges.
If you get used to it, it'd be less cumbersome. Alternatively, consider install wallet on device you used to perform trading.
In your opinion, what are the most secure exchanges in the world, i.e. where can I be most sure that the exchange will not go bankrupt, be hacked, etc.?
No exchange could meet your criteria.