This is a topic with the reef saltwater tank community where the stakes are much higher due to the sensitivity of very expensive corals. Are a reef hobbyist, thus crosses over to my brewing habit. Here are some thoughts.
to repeat myself, small amounts of trace elements can wipe out tens of thousands of dollars of livestock in coral reef tank so the hobby takes this most seriously
1. many reefers report success with walmart RO water. While RODI (Reverse Osmosis Deionized) at 0TDS (zero dissolved solids) is the standard, the Walmart RO is sufficiently low in TDS to use in a reef tank.
2 while a full set up is good, a portable unit under $100 that hooks up to a kitchen sink faucet or a washing machine connection is all you need. I have one such unit that is rated 75 gal per day and I make 5 gal of rodi @ 0 TDS in about one hour each week. Keep in mind this is RODI at 0. An RO unit should be faster.
3. if you have hard water you can add some cheap pre filters to the unit to ease the workload of the RO chamber. Saves you money and time.
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4. Spend a few dollars for an inline TDS meter. They usually are an option with the unit but can be easily added to an existing unit. It takes the guessing game out of when to replace cartridges. I have a dial probe, one just before the DI and one right after so I know how both my RO and my DI are doing and because it’s for corals know precisely the TDS.
with my cheap unit and making 5 gals weekly and an extra 5 gals monthly my unit goes a little more than a year before needing to replace my cartridges. 3 years on the more expensive on (off hand I can’t recall if that’s the RO or 5he a DI)
Summary, Having a unit in the house is a game changer for ease. You don’t need to spend more than $100 for a suitable unit (my TDS meter confirms this). Walmart is reliable if you don’t mind lugging water home.
Fwiw, my local water is fairly neutral. I use the mid points of the most recent water reports and add the salts to reach my desired water profile. That results in, IMHO, extremely good beer, if I do day do myself. Mind you I have Clean slate water on hand to start with on brew days but I find it easier to use the tap (and the calcium sulfur mg etc that comes with it) and adjust accordingly. It’s not precise but I am not going to argue with my taste buds especially if they tell me not to make for extra work.
Hope this helps.
