That’s because it wasn’t finished yet. ROTJ feels quite different to me because the battle is being fought in three fronts, land battle to take down the shields, space battle to get to the Death Star, the psychological struggle between Luke/Vader/Emperor, the fact that it ends with the superficially similar moment of a Death Star blowing up doesn’t negate those differences, and really in a saga called Star Wars it is hard to avoid big space weapons playing a role. Back to whether a second Death Star would have made the empire unstoppable, I don’t think the fact they managed to blow it up means that wouldn’t have been the case, it was incomplete, the rebels were lured into a trap both in space and endor but tge emperor underestimated them, if the Death Star was complete it may have been different. I don’t think the phantom menace should be brought into discussion on ROTJ as it didn’t exist at the time (barely exists now)
