When should I choose to have my forces go all in on an assault...or all in on maneuver...or split them up into a combined attack??
Good question. I'm a relative newcomer to this game so I might not have the most insightful of opinions, however here are my observations:
Maneuver Combat: This seems like the safest and least bloody option most of the time. It probably won't cause any step loses to the defenders, but will likely force them to retreat and hopefully cause some fatigue (which will compel them to re-position to the rear). Also, there is no limit to how many units can perform this kind of combat.
Assault Combat: This can prove to be a more bloody affair, especially for the attacker if the defender has access to a lot of artillery/air support. This type of combat might be a good option if you have numerical superiority. Of course being limited to 4SPs per assault will limit its use.
Combination Combat: Overall I would consider this the best option if you can manage it. For example, if you successfully surround an enemy position (either partially or fully), then you will likely have a large amount of units available for an attack. The maneuver combat portion of the attack will hopefully cause some fatigue to the defenders, which will then impact the later assault. In my experience it will likely take multiple turns of such combined combat to grind-down a well defended position.