When to repot various orchids?

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Originally Posted by ChatteringCat

I have a few orchids that likely need to be repotted, but I have been hesitant to do so for various reasons....

Before going into details, they all sound pretty happy, so I don't think you should be in any rush to repot. Also what Ray wrote about repotting when in growth is important. And I'd add that I prefer to repot when it's warm for warmer-growing orchids, and it's not yet springtime for Germany.

Originally Posted by ChatteringCat

1) The white and purple IKEA phal:....

I would leave it alone, because it is doing so well, and especially because you can see big healthy roots. I am probably of the minority opinion. Crowded big fat roots and big fat roots growing out of the pot are not problems.

Originally Posted by ChatteringCat

...Is it safe to wait until there are no flowers? Will there ever be no flowers?...

Yes. Maybe not.

Originally Posted by ChatteringCat

2) Dendrobium Nobile...accordion leaves at the top of the second cane (I really thought winter rest was a thing... I guess not?)...

Are you sure it's a D. nobile hybrid? Accordion leaves are not common on Dens and it sounds like yours is growing well in any event. The bumps might be buds; this is the right season for nobile buds. Most nobile hybrid leaves are thin and bend easily. D. phalaenopsis hybrids have thick leaves that break, not bend, and their spikes come from the stem apex or axils of the top few leaves. Winter rest for nobiles means temperature drop (cold nights) and no fertilizer. Den phals don't need a winter rest.

Most people grow both in bark. Seramis is fine too.

Originally Posted by ChatteringCat

3) Two different cattleyas (unknown hybrid and Hawaiian Splash) from local nursery: These are both having alot of new root growth over the side of the pot. The unknown hybrid (seriously, the language barrier at the local nursery is annoying, I need to learn better German) also has what might be a new bulb hanging out over the edge of the pot. I am hesitant because these both have a "healthy" green moss growing on the top of the media. Is this there for a reason? I assume the cute little clovers are not something I want to keep, but what about the moss? Questions: What type of media should I pot these in? Straight bark, a Seramis / bark mix, or something else? Do I save the pretty moss

German isn't that hard for an English speaker. English is just a form of German with degenerate grammar and a lot of French words.

The moss is there because they grew in high humidity. It's neither good nor bad. The moss will live or die depending on your relative humidity. The clovers are noxious weeds with exploding seed capsules that can fling the 100% viable seeds 10 feet / 3 meters. People use bark or Seramis for Cattleyas with great results. The most important things with these are bright light and letting them go dry between waterings. If it is already growing out of the pot, it is time to repot. Wait until there is plentiful new root growth. If that is now, go ahead.

Originally Posted by ChatteringCat

4) Oncidium Sharry Baby (from local nursery): ...It sent out two new bulbs close to the pot edge that are about half grown....

Repots when making new roots and when about to overgrow the pot. From reading here it seems Oncidiums are easy-to-grow house plants in most of Europe. People grow them successfully in all kinds of media. They don't like to be dry for very long, if at all.

Originally Posted by ChatteringCat

5) Vanda (random hybrid):

Leave it alone. Most roots normally grow out in the air. The pot and bark are just for convenience and are completely unnecessary for Vandas. As it gets bigger, you can attach hangers to the pot and dangle it in front of a window. The root branch and grow all the time. Remove the bark as it starts to break down. It is fine to have a Vanda in a pot full of nothingness. You figured out they need a lot of water, but they also need to go dry in between. Algae on the roots suggests they aren't drying out enough, but algae itself won't hurt the roots. As long as roots are swinging in the air, staying moist isn't a big threat to root health, but it invites fungus attack.

Water Vanda roots when they go dry. This is usually about once a day. Many people dunk them in buckets.