I don't have an exact amount of time to say to you for how long but we've only been working on this a little more than a week and your numbers have been all over the place. How long as this tank actually been cycling/running now? Looks like you have most of the nastiness sorted with the filter media and pothos but you're last pH reading was a difference of almost a full point from the one before it and that is a HUGE difference, 10x more alkaline. 7.4 or 7.6 (not sure which one was correct there) is better for the shrimp and OK for cories so that's a better place to be then the 6.4 you were at, wouldn't go any higher though. You really want to nail down these readings and get your water changing down to a science since you are messing with chemistry so much before adding any more creatures, those shrimps and snail are fighters. Like we keep saying, patience.
What are you considering artificial minerals? And what don't you like about using them? The minerals we (I) use to reconstitute our RO water is natural minerals, it's not like they are grow in labs and genetically modified.
They are the same things you find in your water just singled out and more "pure". But if you want to cut it with the poison (or toxic waste, sludge whatever you called it lol) coming out of your tap instead of powder minerals, that's your choice and may work just fine.
Depends what is actually IN your water what kind of whole house system you get but yeah there is probably something like what you want out there. Here is my situation for example. I am on an old shallow well that is high in ferrous iron and slightly hard. About 200 TDS 10 GH 10 KH. We used a whole house water softener that removed all minerals and replaced them with sodium ions giving us 200 TDS 0 GH 10 KH, no good for fishes, great for pipes but sucked for showering. I got a RO system and solved my fish problems until I wanted a 3,000+ gallon pond and a RO wasn't gonna cut it. Installed an iron filter with air injection (ferrous iron is 100% dissolved in the water and needs oxygen or other catalyst to make it removable, super layman terms here I know "science" police) that is now the first thing ALL water in my house goes through, then it either breaks off or goes to the softener which we have semi bypassed, now that the iron is gone, to give us the water hardness we like. It breaks off to my pond yard hydrants and auto fill as well as a PVC dedicated line running through both stories of my house just for my fish tanks. Unfortunately none of the fish water runs through the water heater so I have to fill a barrel with water and heat it (usually to room temp only though). So I've been through about ever water situation except city water.
And the lesson is if you spend enough money you can get what you want! lol Water softener was about 5k installed (I think, it's been a LONG time) and the iron filter we installed for about a grand. Ran all extra piping ourselves though.
BTW cleaning your pre-filter and white batting is fine and a good practice to keep. As long as you have other media for the bacteria to grow on. And since you keep them dirty that is where the bacteria will grow instead of the pre-filter and batting.
Excuse the rambling and poor grammar, I'm tired.