Hi Abby. I have never used this produced, but one thing to be careful of, a percentage of the degreasers / cleaners will remove the wax off the exterior of the vehicle. Considering you just waxed the car, I would be very careful in the application and removal of this product. Or you can "spot" the exterior finish have have to re-wax the given areas.
An advanced aged gentleman and Mustang Club of America member, introduced me to Simple Green back in around 1998. We were at a regional event showing our Fox Body 5-Liter Mustangs. He gave pointers of areas to clean and such. His 1992 convertible has won numerous MCA awards.
Anyway, I've used Simple Green since then... I've cleaned engine compartments and floor pans with this bathroom all-purpose cleaner. I typically apply the product with rags, tooth brushes, or paint brushes. And remove the material with a little spray bottle of water and rags.
If something is "really" dirty or in need of a grand detail, I would remove the parts in question and clean them separately from the vehicle.
I have never sprayed a product on / in the engine compartment and then washed it off with a high pressure hose. So, I cannot offer advice on this method.