Starfinder and PF2 use a 'bulk' system, where items have a fairly abstract encumbrance rather than a specified weight. Sounds kind of like what you're looking at.
What you describe sounds a lot like encumbrance (ENC) in Runequest. ... ENC is a measure not just of weight, but also how much space the item takes up. ...
I think that "bulk ENC" systems are the way to go.
When you go purely by weight players will always carry the kitchen sink itself if you let them.
FFG Star Wars used a similar Encumbrance system except that certain gear (like backpacks, utility belts, and combat webbing) increased your character's Encumbrance Threshold (what could be carried before penalties set in).
I would personally allow a pack to increase what you can carry on your person, but not to increase the encumbrance threshold. Heavy is heavy.
Maybe it could increase it slightly - but not to the point that packs can be used to perpetually make yourself "unencumbered". because the additional weight should still count for something.
Coins are usually handled far to generously as well. A $10 roll of 40 quarters is 8oz. you start carrying around 200+ coins that weight really adds up. No one would regularly carry more than 40-80 coins day to day in a world without paper currency...
Hauling the treasure out of the dungeon, getting it back to town, and securing it somewhere safe should be a mini-quest in and of itself.
...
Adventurers should, at the first sign of combat, be dropping their 100 lb backpacks that they're carrying everything but the kitchen sink in... Yet no one does that. ...
This!
Most ENC systems are far too generous.
My go to reference for what PC’s can carry:
Medieval Fantasy Realistic Weapon Loadouts: scholagladiatoriahttps://www.youtube.com/watch?v=DeGe7L5zCkUEspecially going to 28:20 and 30:20 in - you stat to see just how limited you really are for carrying things into combat.
At best only one or two PCs out of a party of 4-6 are actually carrying backpacks into a dungeon where they are all expected to move fast and fight when needed! And your archer is gonna have to rely on other PC's to bring more than 20 arrows into the dungeon.
For my next campaign I'm definitely doing a slot and bulk based ENC system.
Because part of the reason ENC gets handwaved so much in most RPGs is because it is so generous as to be inconsequential.
I am of the mind that if I want my players to proactively engage with the ENC system; It has to force them to make
hard choices as to what they can carry and not be considered "encumbered".
Because being encumbered should involve move penalties, initiative penalties, and ability/saves check penalties all at once.
In my experience players will bend over backwards to not have their PC's suffer penalties of any kind, for any reason.
So ENC systems need to be tight with real penalties for "being encumbered", otherwise players will not willingly or proactively engage with them.