The nutrient profiles in my calculator are copied from various academic publications. Those profiles typically come with less discussion of how they were designed than I might hope, but I assume it's from review of trials of the form mentioned in the other reply to your comment. For more profiles, I've never used anything from

https://cdnmedia.eurofins.com/corporate-eurofins/media/12142...

but I suspect from the author's bio that it's trustworthy.

For probes, see

https://github.com/hydromisc/hydromisc/blob/master/doc/senso...

EC probes should last basically forever if correctly excited (zero net DC). pH probes are consumables, but my $10 probe is still happy after a year continuously immersed. As process fluids go, hydroponic solution is pretty mild, neutral-ish pH and decent ionic content.

I bought the potassium nitrate from MBFerts in combination with other fertilizer salts, so if I'm on a list it's probably "cannabis growers" and not "mad bombers". If you'd rather not use it, then a quick experiment in the calculator shows you can still hit a decent profile with just calcium and magnesium nitrates.