"As a balance concern, however, packs are still limited to one per slot (since packs can have as many resources, or more, than a single slot can hold).

It makes some, but also not a lot sense. The Resource Packer is supposed to be a boost : More resources ine one go for less transport. But using the Resource Packer with the Spelunker is only a resource drain if one doesn't use the drones. Otherwise, it is a downgrade with the Spelunker, Example: if you have an 8 size inventory, and you can only hold A single Pack which for the most part will be between 2-8 Resources together, so it is a downgrade for most of the game. You just put in a lot of resources to upgrade the Resource Packer, only to have less or the same as what the inventory could hold anyway. In late game yeah, it can be a lot more but the inventory does too, so it makes no sense to use the Resource Packer, it makes more sense to just upgrade the inventory, which is probably cheaper.

At all levels, the packer can hold more than a slot, and costs about the same, or less, to upgrade as seen here:
Tier 1: Packer 4 / Slot 2
Tier 2: Packer 6 / Slot 4 Cost: 2 iron + 2 water / 4 iron
Tier 3: Packer 10/ Slot 8 Cost: 6 iron + 3 water / 8 + 16 iron
The packer also lets you combine multiple resources in one slot. For example, you can use the packer to combine 2 water, 1 cobalt, and 1 water in 1 slot instead of 3.
The only downside to the packer with spelunker that I see, specifically, is that the increased number of charges is less useful.