I've just had a read through your history and as far as I can tell you had trace ammonia readings but nothing drastic, did the right thing by changing water, but never really went through a proper cycle.
You're also fully stocked, which means water changes are going to be a pita until you're cycled. Honestly, all you can do at this stage is go through the cycle.
When the water enters the filter, does the tube deliver it to the back, and then it passes through the cartridges back to front and then over the waterfall? I'm trying to remember from when I had one of those filters (I didn't use it cuz one of my kids stood on it and broke it!
)
This is what I would do:
1. Replace the cartridges as you should this time, but place them in front of the little groove think they're supposed to sit in, then buy normal filter sponge and put it behind them - fill the space as much as you can, I know it's only small. Cut the marina cartridges up if you have to - remove the plastic surround bit.
2. Don't replace the carbon - it's a waste of money and is only useful for a few days at most to remove medicine after you've treated the tank for something. However after that it acts as media, so you're best to leave it there for the moment. Cut the carbon cartridge up and place it at the back, behind the new sponge, and next to where the water comes in - that way any bacteria that has built up will hopefully seed the rest of the new sponge.
3. Run it like that for 4-6 weeks, testing water every day/every other day to keep an eye on levels and changing water as necessary.
4. Next time it comes to replace cartridges, just chuck them. Replace with more sponge.
The idea is that by running the ammo lock at the end of the filter you will allow the beneficial bacteria to grow in the filter sponge but the ammo lock will still remove the majority of it
after it has passed through the sponge. Then once you remove the anmo lock you should gave a partially cycled filter, and can continue testing and water changing as needed until you're definitely cycled. I only suggest this as the best course as you're fully stocked and also reliant on the ammo cartridges. If you only had a few fish I'd say, just replace with sponge and do a normal fish in cycle, but you don't.
Hope that helps a bit
feel free to come back with any questions.