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The items need to be equipped for their potentials to kick in, right? It doesn't work just by having it in my backpack.

That's general purpose. For targetted content it can be very lower:
1 Weapon only
3 units + 2 rings
Only Arm and Leg parts (no rear, no weapon camo)
Only Moons, RDR, Triboost, and possibly some...

I'm at a similar situation:
3 weapons + Umbla Stick (4)
3 units + 2 rings (5)
3 Cast parts + weapon camo (4)
Monomate, Dimate, Trimate, Sol, Moon, Meseta Boost, RDR Boost, EXP Boost, Triboost,...

It is actually a lot worse. If your "items that I must have on me at all times" take up 30 slots (equipment, boosts, consumables, etc.), then a 50slot character has just 20 room before needing to...

Gunslash is inferior for the exact same reason Sub units are worse than Rear, Arm and Legs; they're supposed to be equipment that does not have restrictions. It's standard game design for something...

I am not into theorycrafting, but would it be possible for a build to exist that does not take the 5% damage from High Level Bonus? Or is it in every single build the best skill to take?

Imagine if the next Mag skill did this: Adds up all the DEF values of your mag and adds them to S-ATK, R-ATK, T-ATK and DEX.