So I've gone with a bow build, but individually clicking on enemies is pretty annoying, and my character doesn't continue attacking other enemies after he's killed his target. Anyone know of a way to fix this or some workaround?

That´s the main problem of Spellforce. The avatar dont have a auto attack. I dont think that there is a solution for this (cause it´s working as intended).

maybe press control and right click on mouse?

This.

A warrior could easily be used as a tank, keep it in front of your army so the enemies will attack him, while your army will do massive damage to them without being harmed.
The warrior could sustain a lot of damage, so you can leave him there in defense while you manage something else.
Also, it is able to do a lot of damage to buildings, so with a warrior alone you can destroy respawn points in the fastest way possible, reducing drastically the difficulty of the game.
If you also add that the respawn points are active only when you have an actual army, destroying them before you make it is even more easy.

Other classes like mages are really fun to use, but at the beginning are mostly just a support for the main army, until later in the game, when are able to use more powerful spells/skills.

Anyway a mage with healing (or other buffs) aura could sustain effectively an army without the need of you doing anything, so you can leave him covered by your men in defense, while you manage something else.
Also mages with conjuring skills can just summon some creatures, and those will do the defense work for you.