I know the above post is 1½ years old, but I'd also very much like what is being requested, and suggested by you, Kirk.
When my catalogues are viewed from the local SSD with the external Thunderbolt Drives disconnected the load speed is near-instant. Once the external drive is connected, load speed is significantly impeded. So, if when connected to the external Thunderbolt dive, PM would load first the internal SSD proxy and then background load the external psd/tif/raw/jpg then searching and browsing could hopefully be near instant.
Machine: M1 Max 64GB
Catalogues: Roughly 90,000 images
External Disks: Thunderbolt connected SATA disks