Seeing those linked backpack bugs made me think that a different backpack could be made out of this.
It'd be a standalone backpack that can't be improved further (except for adding upgrades) and that does nothing special on its own, but can be linked to another ender backpack by putting them in the same crafting grid along with an ender pearl, however both backpacks need to be empty for that.

You could share the linked backpack to a friend or leave it at the base on top of a hopper/storage solution to handle the items in whatever way you want.

By default the capacity could be the same as the leather backpack, and the name of the backpack could always be overridden to show the id of the link.

The crafting recipe could be something like these unless you think they're too cheap, but please no nether stars.

There could also be a recipe to unlink it by simply putting it in a blank grid and getting it out, and it would go back to its old inventory UID.

We just touched on that topic in the E6 discord, how cool a EnderStorage integration would be. The main idea was simply to save the inventory space of a ender pouch just to port stuff back to the base. A pouch addon" with a small segment behaving like an enderpouch as part of the backpack? (Potentiall using up an upgrade slot.) However you seem to eb already on something similar. Keep up the good work, happy new year.

Actually, I have a slightly different plan - linking upgrade (may need to come up with a different name than upgrade as it's not going to go into one of the slots but rather be crafted with backpack) that can only be applied to an empty backpack of the same tier as the one it's linked to.

This sounds awesome! Could it also work with Sophisticated Storage? Link a backpack to a chest/barrel?

Maybe add a config option/toggle where you can't craft it, but can only find it on those Endermen with the musical backpacks and or as loot in the nether or the end... Sophisticated Backpacks with linking and magnet upgrades are mighty powerful QoL enhancements.

Been a while, but I have thoughts for this as well. The "linking upgrade" could just be called an "Ender Upgrade."

Config tab on right allows for changing "channels," similar to how Ender Storage had three colors. The second you change the channel, the backpack storage changes in front of you. You could optionally turn the upgrade off to use as a normal backpack.

This means that when you place the upgrade in the backpack, the storage is "hidden." Another option is to only allow using the upgrade in an empty backpack.

Here's a mockup I designed:

Important considerations:

Ender Storage capacity: Can you upgrade ender storage? If so, is it channel-by-channel? I suggest all ender channels are immutable and equivalent capacity, perhaps that of a double chest.

Private channels: Ender Storage had the ability to lock the backpack with a diamond, making all color codes personal to the player. I do highly suggest this is implemented, if possible.

Number of combinations: There are quite a number of possibilities. Suppose 3 digits are used: 1000 channels. 4 digits: 10,000 channels. 3 colors (16 color possibilities): 4096 channels. Now consider player count, if private channels are done. Multiply channels by player count. It would be a tremendous waste of storage to keep track of every empty channel. Thus, it may be useful to somehow keep track of unused channels, and write them in when they become in-use.

Actually, I have a slightly different plan - linking upgrade (may need to come up with a different name than upgrade as it's not going to go into one of the slots but rather be crafted with backpack) that can only be applied to an empty backpack of the same tier as the one it's linked to.

The recipe isn't going too expensive, definitely not nether stars. I like the one with backpack and ender eyes, may just tweak it a little bit and include the ender chest in the recipe as well and a bit of leather and string to "sew" it onto backpack.

In terms of any upgrades or inventory all of that will actually get saved to the linked backpack and shared between two or more of these, so unlinking the backpack will leave it totally empty and upgrades/inventory will stay in the original. And of course you won't be able to unlink the original, only the backpack that had linking upgrade applied can be unlinked.