Jim N. answered 07/07/19
Patient, knowledgeable photography tutor
"I'm wondering if **filming with a 4K camera, possibly downsizing to Full HD, will give better detail than filming with a Full HD camera"
Yes, shooting 4k typically provides better detail even when outputting to HD/lower resolution.
".Sub-question (maybe I should make it a different question?)"
Yes, the rest of this is a bit confusing, maybe try posting again with one or two clear questions.