> It would make much more sense to have a laddered commission, for example: $5k plus 10% of the amount over Threshold1 and an additional 20% of the amount over Threshold2

Anything stopping this where you live? I've done this. When they quote a price they will get say sure and your commission halves under this. They have themself cornered as they already told you the price they can sell it (which they generally inflate) so it very hard for them to say 'no' without you countering why would it matter if that is the price that can get.

You have the upper hand pre-contract. Agents really want the contract at the beginning + they actually have 2 jobs according to a agent grind if mine; the first is getting a buyer and the second is lowering the sellers expectation to except offers as if often the case. This reduces the second.