I don't think any loot tables in vanilla Minecraft use the min/max number providers for attribute "amount", but it can be done in vanilla for custom loot tables, for example this loot table I made: https://pastebin.com/Leyu9Td7 . As you can see, the attack damage modifier and the attack speed modifiers are using min/max, meaning you will get a random attack damage/attack speed somewhere between the min/max. I would like if this was supported in Tiers, as it allows for far more randomization and can make things even more interesting. For example, here is the Hasteful tier with the min/max number providers: https://pastebin.com/QRPXHeFM . It would average out to give players the same +10% dig speed, but if you are unlucky you could get only +5%, or if you are very lucky you could get +15%!

Sorry about the messed up formatting in the first pastebin. Not sure what happened there

This is currently not doable with the implemented system

Yes, this one is great and needed!

Lately I have no plans to add such a system.