Before going further down this particular rabbit hole:
I would agree that you need to get your alkalinity up, and your phosphates are probably pretty high, but it doesn't sound like it's really negatively impacting your tank. Is there anything going on in your tank that makes you think you need to get your phosphates down? Are there any changes you want to make soon (like adding in stony corals) that make you want to get your phosphates down?
Your corals are mostly the type that can tolerate higher phosphates, and your CUC is likely large enough that algae doesn't survive long enough to become a problem in your tank, and those two things masked the buildup of phosphate in your system. Those are also usually the problems that high phosphates cause: Algae growth and coral damage. Neither seem to be a problem in your tank.
In terms of husbandry - 25 seconds, 5 a day shouldn't be enough to throw your phosphates that high, especially with how many fish you have in there. I think Avast estimates that amount as roughly equivalent to 2-3 cubes of frozen. What are you feeding your tank? Reef jerky? Pellets? Flakes? Freeze-dried? Some foods are higher in phosphates than others, so you could be effectively dosing your tank with phosphates that way. Have you ever fed heavier?
To decide if it actually is a problem you need to address, you need to figure out what your actual phosphate concentration is (because there are levels where phosphates become a different kind of problem). Based on your results with dilution, assuming you did the test correctly your phosphates are at least 4.5 ppm. So a 50-100x dilution should get you a result that is in the right range for your checker. Yes, you will lose some resolution diluting it that much, but if your phosphates are really that high I don't think it matters if you measure them at 6.73 ppm or 6.93 ppm. You're just trying to ballpark your levels so that you can figure out how much intervention you need to take.
Other than a massive water change (which still won't really impact your phosphates that much because of how phosphates bind), lanthanum is the only treatment that is going to make a meaningful difference in a relatively short amount of time. You do have a tang, so make sure you research how it can interact with tangs before you start dosing it.
