which digital format for old Hi-8 home videos?
Difficult to reply without knowing if you aiming for a full lossless codec or a lossy one can go too. You talk of H264, but this codec can also be used in a lossless way.

If it was for me, and storage doesn't matter I would go for a true (pixel per pixel) lossless codec.

Here, excluding some other "good but insane storage hungry" lossless codecs your choice is between:


FFV1 codec, a pretty good choice for archival purposes.
H264 codec used in loseless fashion, that take less than the above, but is also handled not so well by NLE out of there.


In case instead you want to retain only a good video quality (let's say visually lossless but not pixel per pixel) you can go for an H264 with a crf let's say about 16 or 18.

Obviously is pure nonsense use lossless codecs if you already acquired media with a poor mpeg2. My suggestion is referred to the first video capture itself. So you should capture and save directly in FFV1 without any intermediate steps, then decide to transcode in lossless h264 or lossy h264 or just keep the FFV1 based on your storage needings.