It makes it feel more like a job with all this stuff to remember...
What's the best yet most effective way of tracking the items you've collected/needed for the community center?
I know the game well enough to remember what item is required at which moment in the game, so I prepare for it and try to have the required item available when needed. Thus, I mostly rely on my memory for that, but if you are returning to the game and discovering its new content, this is probably not an option for you.
Here are a few tips that I also use and that might help you:
I usually try to keep at least 2 to 5 samples of each item to be able to answer any request at any time, but this is usually not possible before some time in the farm.
At early stage, especially for Community Center and even more with remixed bundles, I make a heavy use of the golden scroll button in the top of the inventory:
Either by clicking it to see what items are missing, or simply by hovering on items in my inventory and see if the button pulsates, which is a very convenient indication that the item is required in one of the Community Center bundles.
On late game, when there are many items, I use colour-coded chests to keep items sorted: craft items, fishes, sea food & roes, spring items, summer items, weapons... And of course, this colour-coded chests evolve as the farm develops with more buildings and machines. It may take some tries before finding what best suits your needs.