Substrate options?

[200xth]

Aspen is a good choice but pine is cheaper

Also you don't need to worry about mold as long as you don't go weeks without cleaning. I deep clean once a week, and spot clean daily. Never had a mold issue. I've used both aspen and pine.

Same. I much prefer pine over aspen. Doesn't mold nearly as easily, better absorbency, better odor control. I used to keep both on hand, but I don't bother with aspen anymore.