I finally managed to port my own project, Scalismo, to Scala 3 and thereby to test also breeze 2.0-SNAPSHOT.
As you said, using breeze 1.x in compatibility mode did not work due to implicit resolutions, but switching to breeze 2.0-SNAPSHOT worked flawlessly. The project uses the standard linalg operations (Matrix/Vector multiplications, SVD, Cholesky, Matrix inverse, ...) but no sparse matrices.
The only issue I encountered on the way is the same that Darren already reported, namely that using * for broadcasting does not seem to work anymore.
What is your estimated timeline regarding a non-snapshot release targeting Scala 3.0.0?