I have pretty much all my plants in semi-hydro, where I use LECA as a substrate, which is constantly submerged 1/3 in nutrient solution.

Over the last years, I've transitioned over a hundred different plants (including finicky ones like Calatheas, ferns, etc.) from soil to hydro and never had a problem.
Sure, they are stressed in the beginning and a few leaves are getting crispy, but they usually recover in a few weeks and then really take off.

Orchids are different tho.
I have mostly Oncidiums and Phalaenopsis, and Phals in particular somehow really don't seem to like S/H.
Almost all roots are dying off in the beginning, and I've lost a lot of phals compared to other plants.
And even when they survive the first weeks, winter seems to take a big toll on them.

They need to regrow fresh roots first, which need to be directed into the media, and then they really take off. After two years!

I don't know what I'm doing wrong.

Either that's totally normal.
Many of the specimens I got were already pretty weakened when I bought them, and they would have died anyway?

Or, the transition went too fast.

Or, phals need really warm temperatures, and the constantly moist airy substrate is too cool due to evaporation.

Or, there's something special about orchid roots that makes them completely unable to adapt to new conditions.

Or, maybe something totally different?

What can I do to minimise the losses?

The orchids that we (my wife, primarily) have managed to keep going for some years have all been in a bark substrate. Orchids live naturally up in the crooks of trees and so wet feet aren't a common experience for them. We water ours by placing ice cubes on top of the bark and letting it melt in slowly to water. I truly think the roots need to be dry at least part of the time.