Something is definitely not right.
I'm using a GTX 1660 SUPER for all my GPU assisted work in my server, capabilities should be the same as your 1660 Ti?
For reference, I'm transcoding 2 video files with Tdarr and transcoding a 4K DV/HDR file to my phone over 5G
Same Plex file over WIFI and 1 transcode with Tdarr (The other one had finished in the meantime)
And just for shits and giggles, only Plex transcode, same 4K DV/HDR movie over 5G
I noticed your encoder/decoder is hardly working. Are you sure you have the right read/write access to your temp/transcode folder?