Is there a Mastodon client that hides/collapses already read posts? Similar to an RSS client where I can filter the timeline for unread.

If you've got specific accounts you want to follow, you can get an RSS feed containing their public posts.

In terms of native clients, the closest such feature I know of, is that Fedilab (for Android) can remember your position in the timeline, so that you can resume reading.
If you primarily use one timeline, then you wouldn't re-encounter already read posts, because they're in the past from the remembered position.
I have to say, though, that it's not the most reliable feature in the world...

Boost for Lemmy un-bolds* the old ones