@Artan42:
Well, I experimented a lot before settling on what I am doing now.

My first approach in the past: Take the DVDs and upscale as they are. Looked good on live action, but the effects looked (in my opinion) worse because they were already zoomed in.

My second approach (after the 1080p masters from HBO/Amazon became available): Upscale that to 4K. Looked nice, but was only in 4:3 aspect ratio and personally I think they went overboard with their increased saturation.

My third approach was (as you have suggested): Mixed ratios - Live action 16:9 and 4:3 effects from the 1080p masters. Looked strange to me. Then I experimented with digitally / software-based “painting in” the missing elements from the sides. Looked good on static shots, but crappy as soon as anything moved (like stars, ships and such).

So I went with what I am doing now.