Ever heard of the tool GD stash for Grim Dawn? No?
It allows players to transfer items from the ingame stash (also limited like in BL3) to an external database file. You can transfer items from your savegame to the database and vice versa with just one button press, sort, compare and categorise your items and much more. The size of the database is only limited by your drive space but since those files are tiny and drives are huge these days you can basically store unlimited items.
The current iteration of BL3’s engine seems to be limited in how many items can be loaded simultaneously in your inventory and bank. The more items you have in your inv. and bank the more choppy your framerate becomes. There are modded savegames out there that have 200 and more items in them. When opening the bank on those the framerate goes down the toilet (<5FPS on high-end hardware). If you want to see it in action there are some Youtube videos showcasing that IIRC.
I’d love to be able to use something like that but unfortunately, my coding skills are way too limited.