> Can you model this in EdgeQL?

Absolutely!

  WITH
    april := <datetime>'2020-04-01T00:00+00',

    NewCustomers := (
      SELECT Customer
      FILTER
        NOT EXISTS (
          SELECT .orders
          FILTER .date < april
        )
    ),

    AprilCustomers := (
      SELECT Customer
      FILTER
        datetime_truncate(.orders.date, 'months') = april
    ),

    NewAprilCustomers := (
      SELECT AprilCustomers
      FILTER AprilCustomers IN NewCustomers
    )

  SELECT
    (count(NewAprilCustomers) / count(AprilCustomers)) * 100;

This assumes the following schema:

  type Order_ {
     property date -> datetime;
  }

  type Customer {
     multi link orders -> Order_;
  }