350 GPH more than likely less once that cartridge clogs up and it slows down, is barely over half the volume of the tank turned over per hour. IMO and in the opinion of many many other cichlid keepers in an cichlid tank there's no such thing as over filtration, 8-10x per hour is the acceptable normal flow rate. As you admitted yours broke down before they were adults and before you had their full bio-load in the tank, so really you just discredited your whole " My water was perfect" argument, whatever that statement means.
All my recommendations unless someone asks for questions about fry tanks will always be based off adult needs/requirements.
No matter how much flow/ how large of a filter you have you should still always monitor your water parameters. A filter is not a magical device that gets rid of the biological waste it aids in breaking it down by providing surface area for the bacteria to grow and oxygenates the water.
It maybe cheaper to start with but you also have to remember you get what you pay for. The only cost I have to run my ACs and Canisters is replacing the poly fill and I have had the same $4 bag from Walmart for about 2 years.