Can I make m4v files "better" before archiving? (adding keyframes, or lossless optimization, or ?)
No and yes.

For one, you cannot magically pull out more information from the video you have. On the other hand side, you can try to use NVIDIA's AI-Upscaling, which works fine on documentaries and nature, but it turns humans into horrible creatures. The Ai practically guesses what information is hidden between the pixels, so it's not 100% accurate.

But just thinking about the problem at hand should make it clear, that there's nothing "hidden" between blurry pixels. There just isn't enough information in the stream and you cannot simply enhance the video like you see in csi:miami. You can try using post-processing, such as sharpening, upscaling to 4k using Red-Giant's plugins (which are expensive though) and enhancing contrast. All of that is just post-processing though, so if the file's a turd, you'll be polishing a turd.