Ed how soft is "very soft" EPA soft has hardness of 40-50 mg/L (as CaCO3). Very soft comes in at 1/2 that (hardness ~20-30) and alkalinity ~15.
I use RO water but blend back in about 10% of my well water to get similar water quality to your mains.
My phrags are in a variety of systems. Longifolium, pearcei, richteri and caracinum are in SH that I dump and refill every few days. The media is mostly hydroton, but overgrown with moss. The long petalled phrags are in hanging baskets that get watered every day. I recently picked up a lindleyanum and some fisherii this spring/summer and left them in the bark mix they came in. I did at a tsp of "building sand" and a 1/4 tsp of aragonite (calcite) sand as top dresses, and washed them in. These also get watered almost every day, and stay pretty wet. I do feed lightly about once a week as long as its sunny.
I have found over the last couple of years, that adding the building (or play) sand does seem to keep mixes in better condition a big longer. Maybe it ups the drainage a bit.
I guess the 3 items I have potted in bark are getting the closest treatment to what you are doing, and so far they are also doing great.
I don't know if you've been following the threads on reducing potassium use, but in general the problems with leaf tip burn and erwinia have really slowed down or completely disappeared, not necessarily by feeding sparingly, but by selectively reducing the amount of potassium relative to N, Ca, and Mg.
So at this point I would anticipate good growth for your plants. May consider small dosing of calcium nitrate every now and then to add a little nitrogen.