Is HEVC the best lossless compression for video? If so is there a way to convert all videos in my library to HEVC?

If the videos have already been captured and stored in your camera roll, it may not be feasible to convert all of them to High-Efficiency Video Coding (HEVC) directly through your iOS device. The conversion process typically involves specific settings and may be limited by the capabilities of the device. In such cases, alternative methods or external tools may be necessary to achieve the desired format conversion for HEVC.

HEIF and HEVC offer better compression than JPEG and H. 264, so they use less storage space on your devices and iCloud Photos, while preserving the same visual quality. To fully view, edit or duplicate HEIF and HEVC media on your device, upgrade to the latest version of iOS 11 or later or macOS High Sierra or later.

Using HEIF or HEVC media on Apple devices