I think you need somewhere around 442 Koroks in order to max out all of your inventory slots.
The most costly pouch to upgrade is obviously the weapons pouch, and the easiest is shields, by magnitude of cost.
The final upgrade for the weaponry pouch costs 55 seeds, IIRC, with the five or so upgrades before it only going down by about 10 seeds in cost each, so 55, 45, 35, etc.
It's painful to upgrade that pouch, but it's also the most useful/critical of the bunch.
I say grit your teeth and focus on just the Bow and Weapon pouches, then go find the Hylian Shield and make yourself some Ancient shields at the Tech Lab.
Those will last so long that you won't need to upgrade the shield pouch, saving you some seed expenses.XD