How to convert my iCloud Photo Library into H256/HEIC/HEVC
For images, you could use this iOS app to batch converting anything to HEIF:
I personally use this app for my own Photo Library!
Half - Cut Media Storage in ½
https://apps.apple.com/app/half-cut-media-storage-in/id1288721004
For videos, I haven't test anything solid yet. However, I found a blog post https://rienkjanschurer.nl/how-to-make-your-apple-photos-library-smaller-use-h265-hevc-for-your-videos/
and it uses Permute, which looks promising!
https://software.charliemonroe.net/permute/