Dividing rads into zones

jeremy067390

Hi I have a system with two legs that run in opposite directions around the basement to feed the rads. One is a newer replacement of 1" copper and the other the original old large cast. I want to separate a few rads on the copper side of the system. That leaves two rads that I would either have to extend the original leg to and separate the other from, or connect the two sides together and create a loop and run the others off manifolds and pex home runs. Is it incorrect to have continuous loops? Or would excessively long supply and return pipes that dead end cause the last rads to run cold? Sorry if this sounds like jibberish, I'm only an advanced home owner. If you would like pictures or diagrams let me know. Also cost is somewhat of a factor. If not I'd run it all on manifolds and pex