the problem is that the amount of buildings being supported is not always 1, 2 or 3 depending on the distance to the target. in my current game i can support more than 4 buildings which is clumsy because i need to use 2 outgoing routes to other factories and 1 to a warehouse and then 3 from that warehouse to other factories (because the warehouse can also only support 3). that just doesnt feel intuitive at all. its not even fun because it gets complicated really fast.
in addition i actually played many games where i never used trade routes because there is really no point. maybe instead of inhibiting the use of the destination tab remove it from buildings completely. then it would be possible to use trade routes with the truck depot to carry wares from/to factories and train trade routes for long distance. would imho make much more sense. it is always nicer to only have one mechanic instead of 2 competing options where one is unnecessarily complicated/restricitve to force the player to use the other mechanic as well.