Forget about ammonia/ammonium/ions/whatnot for now. It's really more information than you need in practice. I'll simplify this for you, let's just all call it ALL ammonia.
At pH under 7, ammonia is essentially non-toxic. At ph over 7, ammonia becomes incredibly toxic.
Chloramines are chlorine and ammonia combined. Water conditioners that handle chloramines force ammonia into the non-toxic form, and they do this regardless of pH. But this is usually only temporary, lasting one or two days; during which time they expect your biofilter will have had ample time to remove the ammonia that came in with the water change. They are not a long term solution for large amounts of ammonia.
If you have high ammonia levels and low pH, then you're in a very tricky spot. Because if anything you do raises the pH past 7, then all that ammonia becomes toxic. And ironically, many of the things one might try to help will actually make things worse.
For example, if you have a lot of CO2 in your water, adding an airstone will drive off the CO2 and raise your pH.
Or if you perform a 50% water change, you've reduced your ammonia from 8ppm to 4ppm, which seems like a good thing. But if the water change also raises pH enough, then you have 4ppm of toxic ammonia on your hands instead of 8ppm of non-toxic ammonia. Of course, water conditioner will temporarily detoxify some of this, but it may exceed what a normal dose will handle, and it may not be obvious how much you need to add to handle it all. And if your pH doesn't go back down sufficiently before the temporary detoxification wears off, you're in trouble.
I would first remove the source of the excessive ammonia, which I agree is the Osmocote caps. Scoop them right out, including the surrounding gravel, so that you disturb the caps as little as possible in the tank. The caps and Osmocote can then be separated, the gravel rinsed and returned to the tank.
Then start gradually reducing the ammonia in the water through a series of small water changes, keeping an eye on your pH at all times.
Not much fun. But it's the only way, unless you absolutely commit to keeping your tank pH below 7 at all times for the many months it takes for the Osmocote to wear out enough that the biofilter can finally keep up. Which I consider to be a severe restriction. What if you need to medicate the tank, with something that requires 50% water changes? Accidentally overdrain the tank with a Python water changer? Or any other unanticipated circumstance? Better to solve the problem than to have it covered up, in my opinion.
Hope this helps.