Thanks all. Appreciate your comments and suggestions.
I was just curious as to what TWW does to the water chemistry, in this case RO water. I could have the TWW powder tested, but: a) it would be alot more expensive to do, and b) it would not show the water chemistry end result, namely the chemistry of TWW when it is mixed with the RO water produced here in Toronto (I currently buy the RO water, as I have not yet set up an RO system in my home; it’s something I would need my wife to agree to, especially as it would take up cupboard space).
I have been using the Lelit filter out of pure nervous caution (given DaveC’s admonition about potential Lelit system impacts, apparently not just scaling but also on sensors and valves, etc.). But I agree that if the TWW treated RO water has (after testing) a reasonably safe chemistry, it might be preferable to use that TWW water without a Lelit filter than with one. IF I prefer the taste (assuming I can detect a taste difference) from say BWT filtered water or other water options.
Fyi, I had earlier made my own water using a recipe provided by one of the espressos pundits (Grind Science), which included calcium carbonate and sea salt, which I thoroughly dissolved in carbonated RO water (using a soda stream carbonation device) and then let sit for a day, to let any undissolved powder settle. I then mixed this with RO water to achieve a TDS target, and filtered that through a BWT filter to remove any undisolved powders and to add magnesium (which apparently the BWT filter does). I targeted a TDS (after filtration) of about 165. Grind Science said the recipe results in 100 to 200 ppm total hardness, including 60 to 120 calcium hardness (ppm?) and 40 to 80 ppm magnesium. It apparently had a PH of 6.5 to 7.5. So they said. It did then taste pretty good (according to my notes); I had liked the mouthfeel and preferred it to BWT filtered tap water. Then my Breville DuoPro died of supposedly unrelated causes, and I bought the Lelit. I do wonder about the merits of this particular water recipe vs its unfiltered risks (ie without Lelit filter). However using TWW packets is much much easier.