In my opinion, you won't be happy trying to step temperature with 120V. I did that for years until I finally upgraded to 240V and the difference is night and day. 240 gives you four times the wattage for heating. 120V was good for maintaining a steady mash temp in a cold garage in the winter, but I never achieved a mashout with it.
The salts in the wort that produced the extract have been concentrated, so ideally you would use distilled water to reconstitute the original wort. In practice, if your water isn't too hard or alkaline, it will work for extract. If you're not getting enough hop presence in your pales and ipa's some gypsum might help.
I've dumped a couple of batches long ago for band-aid wild yeast flavor. Lately I'm seeing a lacto sourness showing up in some kegs after several months and I keep drinking those until I need the keg. Right now I have and English Pale that is starting to sour and I like it. I am working on isolating the problem.
I had the same thing happen on my HLT thermometer. I had been doing cold sparges before Marshall made it cool. It was 50 degrees off. I keep it out in the garage and I think the temperature swings over 15 years messed with it. I calibrated it to my Thermopen.
Typical brewing salts such as gypsum, epsom salt, and calcium chloride all reduce the mash pH by reacting with phytin to release free protons (acid). You can reduce pH with acid as well. Most folks have some alkalinity in their water, so reducing pH is the typical path. Less choices for increasing pH: sodium bicarbonate, pickling lime, and pretty worthless calcium carbonate. Download Bru'nwater and it will all become clear.
My big sister and her family used to live in Milwaukee. I remember visiting back in the 70's and they took us to Mader's. I guess it was one of those institutions you took out-of-towners to. Growing up in a very Irish family, it was a blast of German culture. Fond memories.
You have very good water for brewing. It would be a waste to go to RO. Just get a carbon block filter and dilute with distilled for pilsners. I just put a loop of metal strapping on my brewstand and drop the canister in that. I can leave it wet through the cool months, but once it gets hot I have to take the filter out and let everything dry. I learned that by having to trash a moldy filter and tubing.
Conicals do have an advantage with the bottom dump valve. You can dump cold break, yeast, dry hops for more dry hops, and so on. Yes, you can use it as a bottling bucket by mixing in priming sugar and bottling through the racking port.