I've been seeing this issue now as well.
php-fpm[4558]: [WARNING] [pool www] server reached max_children setting (50), consider raising it.
I may remove the GPU stats plugin and see if the issue resolves.
Just want to chime in to say I managed to get this working!
Added the following:
Extra Parameters: --runtime=nvidia
Variable
NVIDIA_VISIBLE_DEVICES: (with the GPU ID)
NVIDIA_DRIVER_CAPABILITIES: all
Now both Tdarr and Plex are using the GPU!