I am a big fan of bio balls when using them in a wet dry filter. There design does a really good job of breaking streams of water down into water drops which increases the water to air surface area increasing water oxygenation and degassing various compounds from the water.View attachment 1357710 My plan was to have mechanical filtration via foam in the first bay where the water is entering. Second bay being biological filtration via bio balls. Last bay housing heater and the return pump.
For a completely submerged application I think you can do much better than bioballs. I would recommend a bag ceramic rings with a couple of air-stones underneath them.
I like your idea of filter pads in the first compartment... coarse on top with finer pads underneath. You could also consider a filter sock holder with a filter sock. To me the biggest consideration for mechanical filter media is easy access and easy to remove and clean. The easier it is to clean the more often you will clean it before the organic matter has a chance to break down into nitrogen compounds.