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! 😄 )
I’m going to play around with some files tonight and see truly what Resolve and Premiere will accept. But I also need to make sure that the HDR metadata isn’t tossed in a conversion. Adobe Media Encoder hates raw H.265 wrapped in a MP4 or MKV container. Same with Resolve. I want to avoid Handbrake if I can because it’s a hog. I like ShanaEncoder but there isn’t much flexibility over the options, plus it tosses out some of the metadata as well.
My goal is to get this all back into an H.265 MKV with at least 10-bit HDR