You don’t need edit layers enabled for a multi layer landscape material. What edit layers does is to allow you to add ‘layers’ to the landscape that can be edited (sculpted or painted) separately and are in the end blended together (which makes editing the landscape slow though). Without the edit layers enabled you only have a single layer. The idea for the multiple layers would be to modify the landscape in a non-destructive way (e.g. if you want to place a building somewhere you may need to flatten the landscape where its placed, with edit layers enabled you can do that flattening in a separate layer and therefore still keep the original landscape data in case you want to move the building).
The other option I know of would be to use a smaller landscape (or maybe instead of one large landscape use multiple smaller ones, never tried that though so don’t know how that will perform runtime).