I make my own laundry soap too, though I use a 5 gallon bucket and make it in liquid form. I just feel it works better since I wash everything in cold water.
We have very hard water as well.
I've washed all of our household laundry and diapers in it. After a very long time I did notice a issue with buildup on clothes, but I combat that by doing a wash with detergent every few months or so. You only notice it because towels don't wick moisture as well and diapers will have issues....but a wash once in a blue moon with detergent will take care of that. Also, our whites don't get nice and bright, but then again we're combating laundry from gardening in 100+ degree heat all day and socks that are running around on hard floors that also house 4 indoor/outdoor dogs and a cat. I'm okay with dingy whites, my clothes feel and smell clean. With very hard water a buildup is inevitable if you are detergent free, but it can be worked with.
I'm not sure if using Fels Naptha in your recipe would help with the buildup. I make all of my own soap, so the bar I use in my recipe is homemade, hence no detergent.
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