I have perfected landing an orbital booster back on Smearth in SR1. Technically it's the entire rocket because it needs the command module but besides that, it's fairly simple. That is, until it comes to touchdown...

With the legs from the first game, You can't use them on the two largest engines because the engine is either the same height or taller than the legs. Sometimes stabilizing it with RCS works but eventually it falls and blows up. Something that would fix this would be larger landing legs, like the ones on a Falcon 9. It would allow even the largest rockets to land safely.