As the title says, It would be a very nice QOL improvement to have a tab in our fairy that shares our treasures account-wide, so we can use these so hard-earned items with all the characters in our account instead of having to spend a lot of time to switch them between characters.

Even just having this with the potions would be a HUGE QoL upgrade that I believe most players would agree to.

This is a good idea. Though, after the Magnus, you can use maids to move items back and forth across your inventory and any town's storage regardless of your location. While still annoying, at least this method saves a lot of time.

I'm afraid that doesn't work with potions

doesnt work with any treasures it sucks

Odd that they would restrict treasures specifically. I don't think anyone would grind twice for them, and trying to force players to go through the extra effort is pointless, they'll just get frustrated about it. They should at least allow them in the family storage or something, though your suggestion is much better.