Almost gave up on slideshow movie stuff. AV1 was using libaom-av1 encoder but it can't glob *.jpg just avif. It was 4 times slower than libx265 or libx264 but 3x smaller though. Then decided use list to concat and most problems solved but half the images whether jpg, webp, avif wouldn't make it into the avif video. Well avif Preview opens it and can flip around images...better than a PDF I suppose. Browsers can open it but not mpv. So just using the container mp4 (or mkv of course) worked.
Just did 11,200+ images was 6h 13m mp4 video 720p and final file size only 415MiB (435MB) which is amazing considering the jpg images were 1024x768 (for the most part and their collective size was 1.35GB. So it's like 3x smaller. Using preset -2 would even be a tad smaller in file size but it's like 5x slower to encode though. So generally not worth it.
The key to all this was putting the duration in the file which is 2 seconds...you could change to 3 seconds and instead of 1/2 put 1/3 or for 4 secs put 1/4.
You could magick convert or mogrify but just using a video filter to scale proptionally saves time
mogrify -resize -1x720 -background black -gravity center -extent -1x720 -format avif *.jpg
mogrify -resize 1280x720 -background black -gravity center -extent 1280x720 -format avif *.jpg
seems like limit for mv * somedir is around 10,000 or 15,000 ...anything over that you gotta use for in loop
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