Thank you so much!
Using the chest in place of the spine bone and leaving the chest empty helps. In the end, I just moved the hips bone downwards in Blender to avoid the issue entirely. I wouldn't ever have thought of trying that without your advice!
Edit: After investigating some more, the strange hip bone position was caused by using the Blender CATS plugin's "fix model" function to clean things up after importing it from VRM. This has been fixed in version 0.16.1 of the plugin and it shouldn't happen at all when importing the VRM file directly into Unity.