For reasons that I won't try to explain here unless asked, the effectiveness of a settling tank is related to the plan-view area of the tank and the settling velocity of the particles. Since iron particles are small unless coagulated, they settle slowly. A round tank that is 4 diameters long is 5 times as effective as a settling tank if it is laying on its side as it is if it is upright.
Your settling tank is probably useful as a retention tank to let the oxidation process take place, but probably not as a settling tank. That is confirmed by your experience.
Where you are getting lots of colloidal iron, I would try inexpensive cotton string-wound filters. Cotton tends to expand when wet and becomes more effective with use. I use the tightest wound cotton filters that I can get to meet requirements for removing giardia, and they hold a lot of dirt before getting plugged.
The pleated 1-micron cartridges that I use in the Big Blue housings do a great job of removing crypto and that sort of stuff but are more expensive than necessary for removing iron particles and don't have as long a life on a gallons per dollar basis for removing ordinary suspended material.
What kind/how big/how many housings do you now have? You could probably try the cotton filters.