The fact that keys break (by default) when trying to open a lock ruins the gameplay of this mod. If I am using a crafted key on a chest with a 88% chance of breaking, that means that aside from that being very tedious, I have to effectively throw iron in large quantities at the chest until it opens up. Comparing the required iron to the loot inside usually means you lost a lot of resources, therefore it is very discouraging to even try to open locks. Additionally getting better keys is pointless, as they have a high chance to break right away. So there is no way out of this spiral by acquiring better keys.

A chest with 3 scarce locks would require on average about 200 iron ingots to open, just to give you some "crap" in return. This means in essence that a player will soon learn that trying to play this mod is nothing a huge waste of time and resources.

I therefore strongly suggest to change the defaults of this mod so that keys do not break unless specifically enabled. That way opening locks is still time consuming, but on average the player will gain resources instead of loosing them.

I increased the success rate of picks and keys in v1.13.0

Keys have a 90% success rate by default, unless used on the wrong lock, then they will break (and not all keys are even breakable). Lock Picks (Pilferer's and Thief's) have a very low success rate (as you stated). This was intentionally designed - they are only meant to help open low level chests so to can get better keys. Not sure where/why you are seeing a high chance of breaking for higher tier keys.
As well, you can modify the loot tables for the chests in any way you like, to make them fit your game setup. The default loot tables were designed to be played with vanilla minecraft, not modpacks, as I have no idea what mods someone chooses to play with.

I can however, make more of the key's attributes configurable, so you could tweak the breakability, success rate, etc.

The Lock Picks are what you have in the beginning, and those break on almost all attempts forcing you to waste tons of resources.

Initially I thought that is only until you got the real keys, but then I got some gem keys and a spider key, and used them on the appropriate lock... and they broke instantly. One of those keys at least said "Breakable: no" and broke as well.