[Gromit]

Some amount of debris is normal; but it is difficult to tell from the photo what you have going there.

By first trans service you mean since you've owned it of course but how many miles has that been?

Can you tell if any of that debris is aluminum? Maybe try to separate it from the metal debris with a magnet; (though the chromium from the bushings won't be magnetic so even when separated it will still be a little uncertain).

I don't know the TH400 at all but I am familiar with the TH350 and they are kinda similar. On the TH350 if you start seeing a lot of aluminum pieces it is best to stop running it in my admittedly amateur opinion.

Because the rebuild kits are cheap but you are less likely to be able to put in a rebuild kit if the aluminum case gets chewed up, so you don't want that.

Here is a pic of a TH350 rebuild kit; you can see these are most of the wear parts besides the pump. It would be common and expected to have some debris from the clutches.