I sometimes notice a slightly earthy smell in mine but I don't think it smells bad. Of course I also am not bothered by the smell of fish food so maybe I'm not a good judge since that seems to bother a lot of people. Usually there isn't much smell at all.
I gently squeeze everything out weekly in old tank water and haven't noticed any bumps in my cycle, so it never really gets that dirty. I buy filter floss in bulk and have a refillable bag and can just toss that part out.
Just a thought, but have you tried adding the carbon into your filter? You don't need it but it's supposed to be good at absorbing odors, and with how often you change the cartridges out anyway you wouldn't need to worry that much about it. If you find that it works you can buy it in bulk and use a refillable bag. Only downside is you have to rinse it super well or it'll turn your media black coming back through the filter (the few times I've used it to pull something out of the water I put the pre-filter sponge on the intake so I can rinse it out a couple times a day for the first couple days).