I read somewhere (can’t find the source) that Sixt NL will provide a card for paying at the charging points and that included in your price is free chargjng.
So you have no costs for driving the electric car!
Charging points are plenty in NL, with fast charging arround the highways. Would not worry about making the trip in one go to Limburg or back.
question remains if you have to return the car FULL, but with free charging card that would be strange.
if you want I can give them a call at the local number.
BTW: rented a 530e recently at Sixt Schiphol, and had a very pleasant experience.