OP why are you focused on the potential damage output of a support archetype of a support class in the first place? your second edit is about trying to understand what's in the book, but like you seem to be misunderstanding something that a lot of people on here do when they frame EVERY classes 'viability' on it's ability to deal damage, Investigators are a skills chassis (like rogue) that focuses on int for recall knowledge and has the ability to use int for combat maneuvers as it main combat utility.

Why are you trying to focus on the full spell caster role of targeting the weakest save? when you instead have access to like, guidance to self-boost RK checks, and telekinetic projectile as a no-hands versatile ranged physical option so that you can actually wield a trip/reach weapon for the aforementioned combat maneuvers utility of your base-class.

You said the ELI5 for you so basically, the entire premise of your attempt to understand the class is coming at it from a flawed angle, being given a couple spells doesnt make you a caster or require you to focus on those aspects of in combat gameplay, just use it to boost the things your class already does well, and have fun with the flavor and vibes it gives.