It's no surprise your water tastes lousy. You have problem water and there is no magic bullet or inline cartridge that will solve you problems and give you quality water. You've done some things right and some things wrong and are not satisfied so...

In order to speak intelligently regarding water treatment the discussion begins with the results of a comprehensive water test from an independent lab... especially with well water and doubly especially with problem well water.

hardness, iron (ferrous and ferric) manganese, copper, sodium, chlorine, radon, PH, nitrates, and bacteria to start.

Measurements like "Some hardness, a lot of iron, some radon, acid water" are difficult to plug into mathematical formulas.

Also need to know # of people in the house, # of bathrooms, and SFR of the well.

With that info we can help...

Add in TDS (total dissolved solids) and sulfates to the list of things we want to know for your water.

detailed water testing was done (that is how I know radon). Most areas were acceptable ranges. Unfortunately, tests were done with water after treatment in place. Some (unknown) corrections were performed after failed PH test. I suspect acid neutralizer tank was low. I do not see a TDS number in the report, but a wide range of hydrocarbons and metals were tested for (after water conditioner output)
and results were below measurement threshold for most. Sulfates were 40 was the highest passing score item.

Problem stats:
radon 530 (radon in air was present-has been reduced below 2.0 since then,
PH 6.39
iron .35

Plumbing supply is mostly CPVC with some remnants of copper--apparently, supply lines replaced, with only a few copper remnants remaining inside walls to master shower where difficult to reach. I have had to fix one serious clog with heavy calcium deposits under kitchen. Apparently a section of waste line had negative slope and clogged so severely that had to cut pipe. (re sloped to best extent possible with limitations on reach into areas of basement finished ceiling)

Complicated in that there is an indoor pool which is a major consumer of water. Currently 3 occupants of house with 3 kitchens, 5.5 bathrooms and a small bar sink. No external sprinklers but outside hose taps use conditioned water (rarely used unless new plantings or using pressure washer).

Well has low production rate. Found well drilling record online after bought house. Well drained to pump depth during initial throughput test! Going deeper not a solution since besides cost, deeper would mean significantly escalated levels of radon given geology of area. But production levels are adequate for normal use without exterior sprinkler system and not filling pool too rapidly.

You need to contact the lab that will be doing the tests, ask them how to sample and how to store the sample on its way to them. You need to take a new sample of the raw(untreated) water and get it tested. Nobody can help you without water analysis - anyone who says they can is blowing smoke up your a** and will give you ineffective water treatment, just like what you have. Why spend time, effort and money to get exactly the same result?

Yes, I realize it is extra work on your part. Yes, I realize it is inconvenient. Yes, I realize you would really prefer an instant answer. Too bad. You can get it right. You can get it cheap. You can get it fast. Pick two - you can't get all three.

DITTO to everything that Alice said.

If you want help then we need to know the details. If we rely on your opinion as what is acceptable and what is not then we are wasting our time and our efforts and you'll be just as unhappy as you are now. If you're not willing to invest the time and money to do it right then tall us now so we don't waste our time.

Living on a well is more work than living on a water system. You'll want to make the water nice and safe. You'll have more complicated treatment hardware and have to learn the maintenance required.

We will need to know the SFR of the well cause it may be too low to adequately backwash a filter or regenerate a softener.