I have run some tests. It does not seem to be working.

I would expect echidna to be able to break testAmount without the helper. It needs a number between 1e16 and 1e20, which should occur frequently now in the distribution of the random numbers.

pragma solidity ^0.8.0;

contract EchidnaTest {
    uint256 private constant min = 16;
    uint256 private constant max = 20;

    function helper(uint256 amount) public {
        amount = (10**min) + (amount % ((10**max) - (10**min)));
        testAmount(amount);
    }

    function testAmount(uint256 amount) public {
        if (amount > (10**min) && amount < (10**max)) {
            assert(false);
        }
    }
}

testMode: assertion
testLimit: 100000

This is working now, thanks @rappie !

I have tested this.

As expected, it is noticably slower to break certain properties than before, but it does manage to break them without helpers.

Personally I'd prefer to have only the logarithmic RNG. I believe this to be superior for fuzzing, but then again I'm no expert :)