What I didn’t catch yet is: do you will combine scenes/entire episodes with different qualities (720p, 1080p, 4K)? If so, i would think to hold onto the quality of the majority of sources works best. Less work to do (if everything is 1080p) and someone would decide to upscale everything to 4K. This someone wouldn’t have the hercules task to junk it to pieces and bring any scene to the same quality.
Of course there would be the work to upscale from 720p to 1080p beforehand, but it wouldn’t take as long as to 4k. At least I think, but if I’m wrong I would be happy to get corrected.