but then it would always be an imperfect solution (e.g. shot the first enemy in my line of sight vs. shot that goblin with low hp if it enters my line of sight and ignore the others
If you could target the goblins with slow HP when you use your overwatch (to use the same exemple) it would already be a way better solution.
I agree with you, presseted readied actions is the only practical solution. With the possibility to auto target all (ON), auto target none (OFF) and to manually target, it would be the best compromise.
As Varangian said in another thread they could also rework some spells to ease their use as reactions and couple this with an easy manual targetting system.
Exemple : - Counterspell could just be "a permanent buff" on the caster that remains until it trigger. Trigger the first time an ennemy (you've eventually manually targeted) cast a level 3+
- Feather fall could also become "a permanent buff" on all allies after a character cast it which would be used once per characters when it suit the situation.