The current lineup is fine. Rojas will hit better with experience, and Marsh is improving against LH pitching.
Marsh in LF and Rojas in CF give them an above average defensive OF, and Castellanos is fine in RF with Rojas covering for his limited range. I think Harper has a chance to be a much better 1B than he is a RF.
What they need to do is upgrade the bench while they wait for the next wave of prospects.
Crawford, Ricones, Bergolla, Miller, etc. are 2-4 years away.
Which is why you want Marchan, Kroon, Dunn, Muzzioti to push Stubbs, Pache, Castro and Caves.