I've grown in those kind of conditions that is actually very close to my water conditions. The only way I was able to be successful in soil was with a good super soil like you have. Then I had to regularly transplant into larger pots.
So if I was going out with a two-week seedling stage I would have that in a 16-ounce Solo Cup. Then I would go into a 2-gallon pot or 3 gallon pot for the next 3-5 weeks. Then just two weeks before I went into flower I would repot into a 5-gallon pot.
Doing the above would carry me into about mid flower before I had to introduce nutrients. Then I kept them about every third watering. By then I was on a watering schedule of once every three days.
If you stay in the same pot you will need to do some feeding from time to time and some flushing just because you're going to have so much salt buildup from your background water. Or if you want to you can cut your background water in half or more with distilled or something else to reduce some of that hardness.
If you found a mix that would keep your part per million under 100 on your water then you could probably just grow without any issues.
Another thing I found with super soils like foxfarm is you do need to add calcium and with your background water you shouldn't, but you will. So do that with foliar feeds.
So if it were me I would mix my water to get my PPM under 100 and I would probably water heavier for a couple weeks to see if that would clear salts. Light feeding at the same time every other water and a foliar of cal/mag with a wetting agent like sm90 or a drop of mild dish soap.
If you have access to water like that I think you could finish your grow without too much difficulty.
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