A base layer of particle hair grass, which renders blazing fast and can have different settings for viewport and rendering. This layer should be fairly dense
Have tested this with Geometry Nodes (and also with Particle Hair, but that updates even slower!) and I used Camera culling together with weight painting, but even with only a bit of all the grass rendered, I cannot get nearly enough grass before my computer starts lagging:
So, unless you still think @jlampel that this is doable on a landscape of this scale, I’d suggest we stick to the method above, with only ‘a few’ blades or clumps of grass, strategically placed.