in the only success I had with Linux today, here’s a command I brute-forced to get ffmpeg use my AMD GPU. I don’t know whether I did it correctly, but it seems to have worked.
ffmpeg -hwaccel vaapi -vaapi_device /dev/dri/renderD128 -i wb.mp4 -vf 'subtitles=wb.srt:force_style=fontcolor=white,format=nv12,hwupload=extra_hw_frames=64' -c:v h264_vaapi wbs.mp4