Money is more of a concern than you're allowing for...especially if you want to run a game where encumbrance actually matters. Because there are scads of ways to spend just a bit of money and then essentially obviate all encumbrance concerns. If money flows like water, having encumbrance isn't better than lacking it.
Ah. I keep forgetting that bags of holding are common as dirt in 5e, as I'm in a system where they're as rare as hens' teeth.
I maintain that making bags of holding that common in 5e does a disservice to the game, but maybe I should put that in the "unpopular opinions" thread.
Availability is another. Just because you have a million bucks and no weight limit doesn't mean you can find anything under the sun in Podunk.
To a point, as unless the good burghers of Podunk are themselves very short of supplies there's going to be food etc. to be had.
And I'm talking just the basics here: food, water, clothing, minor gear such as rope. I agree that not every town is going to have an armourer capable of banging out a decent suit of plate, for example, or have available a good spyglass for purchase.
Capacity in some form other than encumbrance can be a third. E.g., as I mentioned before, you can only wear so many weapons, hold so many things, etc. That's not relevant for everything, but it's relevant for some things. Great, you can carry a thousand suits of plate mail--too bad it takes forever to switch between them so there's literally no point. Etc.
You're certainly on to something with your implied inclusion of bulk as a factor, but a slot-like system is really just encumbrance using a different measurement.
In my own system I kinda wave at this for armour: each armour type has two listed weights: a "worn" weight for when it is being worn and a higher "carried" weight for when it is not, to partly reflect the bulk and inconvenicnce of carrying it. I got this idea after looking at hockey players - their bags of equipment are far more awkward to carry around than that equipment is to "carry" while being worn.
Spoilage and other timing-based mechanics (which I alluded to previously.)
That can be a factor once you're out in the field but doesn't (or shouldn't!) affect what you can load up on while in town. Wear and tear on things such as armour, shields, backpacks, etc. also falls into this category.