I’ve made an offline rosbag to rosbag2 converter that supports all default messages in ROS1 and ROS2. And for custom messages, you can use ros1_bridge’s instructions to create the mappings.
If you’re interested, we maintain rosbag2 in the Tooling Working Group, and this feature seems quite valuable to design this into “official” channels and iron out the technical difficulties that you’ve had (such as requiring separate workspaces). If you want to join a meeting and start discussing next steps on that, we would welcome you - though maybe we’d want to wait to get started until after the Galactic release.