I duplicated the project, disabled Interchange and all the required plugins, tried it again, and Pose Driver Connect was correctly able to set up all the poses and animations in 5.5.3 with INTERCHANGE DISABLED.
Now I should be able to migrate everything into the proper project, so I guess I solved my own issue. This is a work-around that wouldn’t be needed if Epic would just properly test out their own huge changes, like reworking their entire import system.