‘Bowing out’ the subdivisions can be helpful to improve rhythm for both left and right hands.
We often subdivide in our mental practice by singing '1 and, 2 and'. You can also sing words that have rhythmic patterns such as 'caterpillar' for groups of four or 'elephant' for triplets. The Kodaly approach has a clear structured approach to this form of rhythm training.
Once we know what the rhythm sounds like we need to ensure that our playing is following the rhythm we hear in our heads!