egpus have to be explicitly enabled to be used in X config:
https://forums.developer.nvidia.com/t/internal-display-freezing-on-startup-with-egpu/170468/4
You will have to write a script that always does the remove/add action on startup, e.g. using a one-shot systemd unit running before display-manager and gpu-manager.
Addendum: it’s often difficult to get the timing right since the nvidia driver takes some 2 seconds to initialise. So if X starts up before that, the nvidia gpu will not be used.