In game, you can wear your backpack and that is ok.

But you can also add multiple backpacks to your vanilla inventory and fill those with items and makes kind of a overkill exploit.

Can you make it so the backpack you wear is the only one you can/use/fill? or maybe you can inspect other backpacks but not add/remove items?

Well, technically all other backpack mods probably allow to have full inventory of backpacks. I don't think that would change anything though. as for now I do not add this feature

I can't remember which Backpack Mod it was, but one had the solution to add a slowness effect. You was able to configure a maximum backpack amount for the inventory, If you execeed that Cap each Backpack adds a configured slowness debuff.

Found it:
https://github.com/P3pp3rF1y/SophisticatedBackpacks/blob/debfc9c3136d9fe1434173716dc0d0909a7c715f/src/main/java/net/p3pp3rf1y/sophisticatedbackpacks/Config.java#L148

Being able to wear a bag, and put many other filled bags in your inventory, is not very sensible. If you want that, use shulker boxes. But I suppose you could add that functionality to the mod using an option "realistic-backpack-mode".

I agree with this. Similar to real life, you can wear one bag, but not another. I would also be good with seeing interactions with a 2nd bag only work if that 2nd bag is on the ground.

I think it is ok, it fucntion like bundle, as in vanilla you can also bring so many bundle right?

https://www.curseforge.com/minecraft/mc-mods/travelers-backpack/files
https://www.curseforge.com/minecraft/mc-mods/travelers-backpack-fabric/files/all
added - can be turned on by config