Sponges are just as good a medium as ceramic. They are both mechanical and biological media.
The filter in the Primo 70 has two sponges. If you really want to use a ceramic medium, I would leave one sponge in there to catch the debris. The bottom sponge is first in the direction of water flow, so leave that one there and replace the upper one with a mesh bag full of a ceramic medium.
However, unlike the larger Juwel tanks, the filter seems to be held in place with suckers rather than silicone blobs, which makes it a lot easier to change the filter. You would need to buy a heater bracket if you did this as the heater fits inside the Juwel filter case.
The Eheim Biopower filters have sintered glass (substrat pro) rather than ceramics. The smallest one is suitable for 70 litres, and this has two baskets, one with sponge to collect the bits and one with substrat pro.