I don’t use Premiere, but AFAIK it should load lossless sources; so, it will be better to convert the UHD HEVC into lossless, instead h.264 (unless you want to use its lossless version, of course!)
That’s a gigantic project; hope you will success; curious to follow your work in progress, because I think we all, sooner or later, will be “forced” to make our project in UHD (and I can’t think about 8K! 😄 )