You're doing very well. Orchids grow very slowly compared to a lot of other plants. You need to be patient.
I suspect reverse osmosis water is cheaper than distilled? Often you can buy RO water at an aquarium shop if you bring your own bucket or jug. You can also install a reverse osmosis unit below one of your sinks.
Many people use KelpMax about once a week when rerooting plants, and about once a month for established plants. My bottle label recommends 2 teaspoons / 10ml per gallon / 3.78 liters of water for use as a drench.
"Very weakly" is hard for me to interpret. People who fertilize at each watering often dilute fertilizer to yield a nitrogen concentration between 15 and 125 parts per million, depending on how often they water. There is a calculator you can use to figure out dilutions. It is on the Web site of the man who sells KelpMax, Ray Barkalow:
Nitrogen Management Calculator - First Rays LLC
50% relative humidity is fine for Phal hybrids. Lower is not much of a problem, either. Not many homes with resident mammals go much below 30%. I would not worry more about humidity.
It is possible to have too much air circulation. Phals are not adapted to flapping in the breeze, the way seashore orchids, like Tolumnia, are, or treetop orchids, like many Cattleya. Increased air circulation at relatively low humidity will cause increased transpiration (water loss) from the plants' leaves. This is not a problem for a plant with a healthy root system, and an owner who waters properly, but a plant with few roots may struggle to keep up with water losses. That is why people recommend trying to root plants in enclosed spaces.
The orchid transplanted from S/H to the bark mix will not get as much water as it did in S/H, and the existing roots are adapted to S/H. Whenever a plant transitions to, or from, S/H, it will lose all the existing roots, because they were adapted to the previous method of culture. It will grow a new set of roots adapted to the new method of culture. That plant won't look happy for some time, until it grows new roots, but it will eventually adapt, grow and bloom.
The plant in sphag and bag can go in light appropriate for the other plants.
Most people use kelp at a different time than fertilizing. However, Ray sells (sold?) a product containing both fertilizer and KelpMax, intended for those who don't want to mix multiple solutions.
More holes in the pots is better. The faster the medium dries out, the less chance the roots will sit wet long enough to rot.
As mentioned at the top, you're doing very well. I expect you to post flower photos next winter/spring when your Phals bloom.
My set up. My challenges!
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