You should decide whether you would like built-in visualization tools or a coordinate frame management system, or something a little more bare-bones.
transformations.py is bundled with ROS tf and therefore is commonly used around the ROS community. It is primarily useful for constructing raw rotation matrices and homogeneous transformation matrices, from various rotation representations and translation vectors. It's only external dependency is numpy, and it can also be installed through PyPI if you are not using ROS.
When you introduce the concept of coordinate systems / frames, I recommend you study active vs. passive transformations (if you are not already familiar) and seek out a package which makes this distinction in the API. Anytime you interact with raw transformation matrices (create, multiply with a vector, etc.), you will need to know which paradigm you are dealing with.
For the above distinction and for visualization options, I would recommend looking at pytransform3d, which also uses numpy, but provides some ROS-like transform tree management, visualization with matplotlib and open3d, as well as excellent documentation (IMO).