I think if Brandeis is requiring ED for coach’s support, they should give you a financial pre-read. It’s not like it is a big secret how much aid they will give to an athletic and it is just a timing thing whether they tell you now or tell you in Dec/Jan. Have them tell you now.
This is not an unusual request. Tell the coach you need to know before applying ED to compare it with other offers. The coach should understand that it is a $20k or more x 4 years decision. That’s a lot of money for most people.
You can make it clear that Brandeis is your first choice but finances are a consideration.