Take your pic out of the variety of botched epic things that you have enabled.
Nanite? Lumen? Etc.
I would suggest like always to finish your landscape editing, export everything, and turn it into meshes.
Landscape is 100% part of the problem for performance. Eliminating it will usually ned a 20% frame imcrease, even if it is a lot of work.
Moreover, since you are concerned with tris count - your meshes can be analytical. The landscape cannot. The tris distribution is 1 to a meter using the default x/y=100 scale.
On deseet areas and flat surfaces nearly all the tris are completely wasted…
Try Hourini for world generation. It seems to have tools that actually help.