If I understand what you are asking, I'd start with fetching blocks and then filter them by their pool name. When you find a match you can output the transactions for that block.
//get latest blocks
$url = "https://api.smartbit.com.au/v1/blockchain/blocks";
$fgc = json_decode(file_get_contents($url), true);
$blocks = $fgc["blocks"];
Your proposed approach is not practical:
Users might change the amount, or omit the fractional part
The fractional part might represent significant value - a user with an ID of 1234567 will have to pay a minimum of 0.01234567, well over a hundred USD at the moment
If users are withdrawing from an exchange or some other wallet without much control over the ...
Evebtually I decide to do it the curl way.
By having my express app to make a simple GET http request to the bitcoin node, I managed to get the job done.
Doing this with the request npm module is súper simple.
It doesn't solve the issue with the JS api, but allows me to properly leverage on the éstimate fee feature of Bcoin.