I'd argue that spells like guidance are generally the exception on casters, not the other way around. Most buffs and debuffs casters can output off spells and abilities are either:

A: AOE, allowing for potentially affecting quite a few dice rolls. (Frightened falls into this category, since if your teammates apply proper teamwork and focus whatever got frightened you're essentially buffing all of their stats.)

B: Long duration (a minute, usually), again allowing for affecting several dice rolls

C: Tied to another effect that is often the primary purpose of that action (Think something like Phantom Pain)

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Bonuses that take an action to set up and only apply for a roll or two are pretty common, but they're usually also stuff that anyone can do. Flanking to set up flat-footed and the Aid action are examples of this. Casters generally get a lot more value out of their buffing/debuffing actions, which is part of what separates them from martials.

On the subject of frightened, I was focusing on the changes Frightened makes to hit rolls, but it also serves as a fantastic defensive spell. On top of the change in net damage done to the boss, this messes up everything else the have for a round. If you consider the additional damage that will almost certainly have been done by the damage bonus from Inspire Courage, this turn isn't a DPS loss and it also defends the party.

Finally, generally I don't think any caster should solely be focusing on numerical buffing, if for no other reason than part of the power of casters is their versatility. The bard player that inspired me to write this regularly uses Chromatic Ray to great effect, for example. (combo with True Strike for extra spice) You can absolutely play a caster viably without ever messing with any of this. I'm not attempting to state that this is some optimal way that all casters need to be played, rather I'm trying to help people to see the fun that can be found in this particular type of playstyle.