For example, If I have a long term alliance I can invest on a long range trade route. But if I want to keep trade routes flexibility, I should prefer to set a short trade route. Am I correct ?
Basically, yes. You probably want highest yield destinations, unless you need the trader to create strategic roads somewhere or establish a trading post, which will open more destinations.
Be aware, though, that this UI shows incorrect information, as concerns the duration of the route, and is misleading.
The numbers next to the clock symbol is not duration in turns, but the shortest distance to the destination in tiles, ignoring all geographical obstacles. The actual route displayed can be very different. On normal speed and size maps the shortest possible route will complete in no less than 22 turns.
The trader must complete a full journey to the destinaiton and back, moving 1 tile per turn, and if they spent less than 21 turns for the journey, they go again for another full journey. So, with the settings mentioned above, going to a destination 10 tiles away (of actual itinerary) will land you with a 40 turns long trade route. Destination 5 tiles away - 30 turns long route, while a destination 6 tiles away - 24 turns long route. The shortest route will be to a destination 11 tiles away in real trip terms - 22 turns.
But you have to count the tiles of an actual route yourself, UI is useless here.
You can read more about it in
this guide.