Wow. Sounds like maybe there are multiple things at play here.
I would venture a guess to say that the death of the tetras created an ammonia spike, and that is what is killing plants. Sword are hardy plants, but sensitive to drastic swings in water chemistry. Mystery snails are a small species of Apple snail, and may occasionally graze on plants, but they normally do not destroy plants that fast. Or, at least, I have never had them tear up a tank fast. In fact, I personally have never had them eat plants.
Weekly water changes will not overly stress out the fish. It's normal procedure for those of us who dose fertilizers using the EI method. Heck, i used to do 2 50% changes on my shrimp tanks back in the day.
Unfortunately, there's no way to know now why the tetras died. It may just have been coincidence. Did you see all the tetras when you added the snails? Possibly one died, unnoticed, causing an ammonia spike which finished off the rest of the tetras?
Oh, and get a real test kit, those strips are garbage lol
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