PIE memory doubling are expected I would guess, I would assume that it recreates all Assets within the Level and loads it into the Memory, that way you are able to make changes during PIE, which are not represented in the Editor after stoping PIE.
However it would be really good to know what’s actually causing the Landscape memory tripling. 1,3GB per 8k Landscape would be alot more reasonable than 4GB.
As for the work flow, you might want to look into Levelstreaming Level Streaming in Unreal Engine | Unreal Engine 5.2 Documentation - it actually helps a lot for huge Landscapes. If you select Window > Levels you can dynamically disable and enable whatever levels you would like to be loaded into Memory.
And yea, if PIE wouldn’t be that memory intense, that would increase productivity alot (loading in huge files like the landscape take quite a while) - I’m not quite sure how easy this would be to implement though.