can we have an option to only be able to carry 1 backpack at a time (or configurable amount perhaps)?
i feel this would be useful for balancing

Technically doable, and personally I like the result of the idea, but a side effect is changing the containing inventory of a backpack would become a hassle. Imagine someone having 5 backpacks for different occassions but moving, means they would have to run five times alone to transfer all the backpacks.

Two other solutions are better imo.
The first one is to only allow access to the backpack in the backpack slot from trinkets.
The second one I am probably going to use is to change the recipe of frayed backpack to require a shulker box instead of a chest, making aquiring several backpacks moderately costly.

those 2 solutions, though great options, don't offer the ideal i'm attempting to find here.
i don't think travelling back and forth to transfer multiple packs would be such a hassle, players have to do this already for multiple chests full of items anyway.
to be more transparent i'm wanting something that allows a slightly larger, maybe configurable/expandable amount of inventory for the player without giving them the option to carry multiple inventories..
something like the enderchest inventory would be best, an inventory "shared" across items like that would mean making more of the particular item wouldn't mean creating more inventory space.
thus still keeping shulker boxes relevant.
currently using the backpacks from campanion, they're pretty awesome and nearly completely hit the mark for what i want, the inventories can only be accessed when in the chest slot, when removed from the chest the items get ejected and spill out to fill the player inventory and the floor around them...
great but it doesn't have configurable amount of slots(expandable) and, as i say, they're required to be worn in the chest slot, so wearing a full set of armour cannot be done in this case. cool for particular cases but i like options :)

ah, I forgot enderchests. With enderchests it would work. But could still be tricky to implement.

something like the enderchest inventory would be best, an inventory "shared" across items like that would mean making more of the particular item wouldn't mean creating more inventory space.

then maybe you want to use linked storage instead

if linked storage just had a single bag, was assigned to only the player that created it and didn't give the option to make as many different extra inventories as you please then perhaps it would... feeling the point here is still being missed. i'll try to think harder about how to explain myself and get back to this when something which more accurately expresses my desire is forthcoming

Your point is understood, I am just exploring alternatives. e.g. there could be a config entry so linked storage only allows one color per player or something

that could work... i found this which appears to pretty much have the essence of the idea: https://www.curseforge.com/minecraft/mc-mods/enderbackpack

you could theoretically disable all backpack recipes except the ender pouch, which is kind of ender back pack, just permanently linked to the players ender chest and not extendable beyond its 27 stacks, besides putting shulker boxes into it of course.

yea... again though, i think the point is being slightly missed... the idea is to have something "like" the enderchest inventory, not using that exactly, there are already a multitude of ender chest linking mods which do this :)

having a separate inventory from the ender chest, so they can be used independently, and being upgradable whilst also being able to be used whilst not being worn in the chest slot or in a trinket slot would be nice. that way no matter how many packs a player makes they're not expanding their inventory every time.
the problem with so many of the backpack/bag style mods is players can just use however many of them they please, taking their inventories from 9x3 slots to 9x3x(9x3) with little to no effort, some even allow them to be stored inside shulkers... that's something like 272727 (not including hotbar slots) allowing the player to carry over 1 million items/blocks isn't an issue for other people maybe but i don't want players doing that.

i'd like players to bee able to work towards more inventory space (additional slots which aren't otherwise obtainable via vanilla play) but it'd like that space to be finite to a number that i can configure.. i've already made shulker boxes super unobtainable for most players so i don't see them as a problem though having an option to list items which either can or can't be placed within that extra space would be helpful to others no doubt..

when i brought up this idea i was thinking, because there wasn't such a thing available i don't even mind if i can't use it, it's just a useful thing for people to have access to in general, i'm building a modpack which is 1.16.5 and fabric, i realise this limits the chance of it becoming available to myself fit for purpose for my pack but i didn't mind.
now it looks like someone has finally made the thing via the mod i posted earlier, idk i've not used it myself...
now i'll specifically ask if it can be done for 1.16.5 fabric expecting it can't and just leave it there :)
i'm fine with it