I will address water change downcopy, but first, when did you last use the Metal Free, and how much? Metal free is actually citric acid according to the MSDS, not the kind of "HEDP" based metal sequestrant we normally recommend at TFP (Jacks Magic or Metal Magic).
Citric acid can consume/fight chlorine and could possibly be contributing to those fast FC drops.
Regardless of why your chlorine is not getting up to level (chemical interaction or organics or both), if you're not changing your water and you're going to move forward with your slam, you need to keep adding chlorine until you reach your slam level...not over time, if its dropping immediately, but in a single session, where you ad, read, add, read, add, read until at 16. Then check back in an hour or so.
The
SLAM Process will ultimately clear/sanitize your water as a baseline start over (though it ay tint water from oxidization of metal), but going forward with well water means less reliance on pool store elixirs you've been using and adopting the TFP way of water care...where you very deliberately only add base elements to reduce unintended consequences. You sound ready for the wee learning curve that involves and this forum is a great tool for getting there.
So, first off, the decision to truck water is a blend of convenience, cost, and only sometimes, necessity

But the trick to figuring out if you can manage the current load of iron is to have a clear idea of your iron level in pool, in your well, and out of your hose if the latter is filtered.
If you know your well level, please report back. Also either try to get a read on the pool level either through purchasing Lamotte instatest iron strips or checking at the pool store (please don't buy anything there or listen to their well meaning but frankly guessing advice). Your pool read might not be accurate at the moment with the Metal Free, but its worth a shot.
Once you have an idea of your iron read at well, and possibly in pool, there are steps to managing a pool with iron.
I will post back more when you give more info -- the steps come AFTER the slam is complete:
Overnight Chlorine Loss Test
But I will give you a taste of what it takes to successfully manage well water with iron and still have a sparkly crystal clear pool...you kinda need to know this stuff anyway even if you start with fresh trucked water because you're on well and metals accumulate over time
Post Slam iron management options:
- If water is tinted brown, extra filtration with
The Slime Bag | The Easiest Way To Maintain Crystal Clear Pool Water. attached to a pool return will help remove rust color, but sometime proper metal sequestrant alone will also clear, as will your filter over time
- For stains, high dose of Metal Magic works for many after a sponge test (
http://www.proteampoolcare.com/images/uploads/MetalMagicSpongeTest.pdf) then maintaining sequestrant with much smaller weekly doses...this is easier than the Ascorbic Acid treatment for stains, IMHO, but that is another option for further future discussion
- using TFP water care method of maintaining [fc/cya][/FC/cya] with unstabilized products, eg bleach, not dichlor... to avoid ever having to shock/slam again will help -- shocking oxidizes metal and consumes sequestrant...that's why this happened when you threw in the shock combined with the well water top up...and why you aren't supposed to shock after a metal sequestrant
-- keeping ph lower, eg 7.2-7.4 to avoid staining
And
-- pre-filtering your top up hose for pool refills, either with something like PreFresh filter (disposable, good for 8,000 gal) or attaching a big blue 10" filter housing with a filter that gets down to 1 micron (costs same as prefresh but can change filters forever, so cheaper in long haul and better water pressure) -- this is to help reduce SOME, but not ALL of an iron load...because nothing completely prevents or removes iron. Ever.
In my case, with a well at 2 ppm (iron stains at .3 ppm) I actually fill from a dual softener plumbed to my pool spigot. That gets it down to a range of .3 to .5 ish (it varies)..then the big blue Pentek filter ataced to hose seems to get down to about .1 ppm for me. At this level, I can now go weeks without adding sequestrant.
Hope the iron tutorial gives you some kind of baseline to decide on the water haul or not...even if you get new water, filter your additions to keep it as metal free as possible
But do pst back with your details and slam progress for immediate interim help. This post was meant for big-picture, longterm in order to help you decide.