Suggestion (Required)

Since there usually wind up being 5 tiers for things like this (leather, iron, gold, diamond, netherite), I think that would be cool to add 2 more tiers to the Bags of Holding. The recipes might need to change if you do that though. With the Reinforced Shulker Boxes mod, the netherite shulker box has the same size inventory as the diamond one, but a different accent color, and is immune to lava. If you don't want the bags to get larger inventories, for netherite bags of holding, you might offer a similar lava-immunity power instead of increasing the inventory size.

Speaking of special powers for the bags, have you considered making the pags work with shulker box enchantments like EnchantedShulkers? If you extend shulker boxes, which I think you do, they might already work, but I have not tried it yet. I think there are only 1 or 2 different mods that add enchantments to shulker boxes though.

A 1.20.4 version also would be awesome, really need a bag/backpack mod for the most recent version