Marquand had plenty of input. Check out J.W. Rinzler’s book on the making of the film.
Perhaps the most important decisions he made were to cast Ian McDiarmid as Palpatine and to bring back Yoda. He also had his choice of Director of Photography and editor - hopefully the different look and pacing of ROTJ compared to the previous two films is apparent.
One of the highlights of Rinzler’s book is a transcript of a conversation between Lucas, Marquand and Larry Kasdan. Most of it comprises an argument between Lucas and Kasdan - you get the impression Marquand is listening patiently and then suddenly he cuts in and chides them for their behaviour. No mere servant.
It’s clear that Marquand was ‘inexperienced’ (being kind) or ‘out of his depth’ (being harsh) regarding the Star Wars shooting method, which in the early 80s was still unusual and more technically demanding than other franchises like Bond or Star Trek. But he was not inexperienced when it came to dealing with actors, as he had been one himself. He also knew how to shoot quickly and economically - that, plus his willingness to relcoate to the US - made him an easy choice for director in Lucas’ eyes.