Personally I bookmark the last read post, and upon reading, bookmark the new one and unbookmark the old.

A feature from RSS readers known as grouping, where instead of a wall of posts you could see an overview:

Mechanical Keyboards (5)
Lemmy (3)
Not Just Bikes (2)
Another Community (7)
Some Community (1)
…
…

For each community you see the number of unreads. If you click on one of these entries, you get the posts of that community of course, ideally being able to set up a “read” marker that “I’ve read until this post here”.

One practical idea to not require API changes for this feature could be to use bookmarks as read indicators. It’s more in the “quick and dirty” realm of course…

A dedicated community has been created for this, see https://lemmy.ml/c/librephone (I just saw this community and joined myself)

I disagree with the strong sentiment in most other comments. It’s not a total crime to cross-post individual things manually. For example, I’ve recently de-facto cross-posted this, and it was fine: https://lemmy.ml/post/37434570

That being said, I would absolutely not automate such stuff. I think having to do it manually is a good barrier so that you’d post truly interesting to you stuff, and skip the rest. I wouldn’t even participate or encourage software that tries to automate this.