The problem is easy to define. The new washers pump out too fast for the drain line to handle. Spend a fortune and a lot of grief modifying the piping or... do the easy thing.
Reduce the pump-out rate by a third to a half.
Get to the pump (may be difficult), remove the hose connector, insert a washer with a small hole in the hose, reclamp the hose.
Difficulties include loosening the hose adaptor and reattaching. This is easier with a nut driver than a screw driver. Also, use a flashlight to see the connector.
Make sure the washer is big enough to hold in place and yet has a small enough hole (3/8") to slow down the flow. You can use three washers to combine these features, and glue them together. Sandwich the small hole washer in between two large washers.
The advantage of doing the restiction at the pump is it doesn't increase the pressure on the hose. If your hose is strong, you could jam something into the hose to slow down the flow.
Cost of solution is a few cents for washers, and a little time.
Hmmmm..... I wonder if Thomas Edison or Macgyver would of thought up your solution !!!
Cut it 3 times & it's still to short.
Inventor of the Miter Master Plus.