Don't know if it's still possible but at one time a passenger with a BE seat could purchase seat assignments beginning 7 days before departure.
Have no idea if that price is less than the $100 charge the OP is currently facing.
No guarantee that windows and aisle seats are available at T-7 days but definitely more likely than at T-24 hours.
https://thriftytraveler.com/guides/a...omy-pick-seat/