Hi, It's only a couple of months I'm using SD, but I cannot find a way to set a route to my alternate.
When I set an airport as alternate, SD draws a single line from the destination to the alternate, no way to set waypoints on it.

You're not missing anything. You cannot plan a route to your alternate, because we don't know at what point you may decide to divert. You can of course plan routes and save them separately, and load them on demand, or plan on the fly if it becomes clear you need to divert.

Good day,
May I join the topic by adding a question…
If my alternate is a nearby airfield SD calculates DCT and therefore way too short. It would be a great help, to make planning realistic, to at least have the possibility to specify a minimum distance to be considered for the diversion.
Thanks
Andy

PS: worst case should be considered for diversion, e.g. diverding after preceding traffic blocks runway, or weather makes landing not possible