I’d recommend following Visual Tech Art’s tutorial for Tri-Planar Projection. Based on what you’ve described already doing, it sounds like you know your way around the material editor, so his tutorial shouldn’t be too difficult to follow. IMO he does a great job showing you what’s common, why it’s “bad”, and what you should do instead.
IIRC, he does specifically account for landscape, even creating some variants which work better for landscapes than traditional Tri-Planar does (Enta-Planar, I believe is what he called it).
He also balances the performance of Tri-Planar by calculating the projection in the UVs of the texture, so he doesn’t have to sample the same texture 3 times (like in traditional methods). This also allows you to use the same UV set for multiple textures (eg: Albedo, Normal, ORM), whereas in traditional Tri-Planar, you’d have to repeat the same process for every texture. This means it’s scalable, particularly useful if you have many landscape layers requiring world-projection.
He even takes it a step further in another video where he focuses on assets, tackling problems like animated meshes and scaling, if you’re interested.