Have you tried other options?
According to the hwdec section of the mpv docs:
nvdec and nvdec-copy are the newest, and recommended method to do hardware decoding on Nvidia GPUs.
Also in the video-output-drivers section on vdpau:
vdpau (X11 only)
Uses the VDPAU interface to display and optionally also decode video. Hardware decoding is used with --hwdec=vdpau. Note that there is absolutely no reason to use this, other than compatibility. We strongly recommend that you use --vo=gpu with --hwdec=nvdec instead.
To check if hardware decoding is working and which one is being used, just press i:
i and I
Show/toggle an overlay displaying statistics about the currently playing file such as codec, framerate, number of dropped frames and so on. See STATS for more information.
You’ll see a Video entry and HW: <codec name> besides it if everything is working fine.
PS: You can put multiple codecs to fall back to when one can’t be used, for example:
hwdec=nvdec,nvdec-copy,vaapi,vaapi-copy,auto