How about building a grave under the big tree? Is it good for feng shui?

You'd better not build a grave under the big tree. Absolutely not. The reason is actually very simple. Have you considered the problem of "root"? How did you avoid it? Do you think it's good to stick the roots into the urn or break it? Don't consider the vertical direction and geomantic omen, just for safety, you will have a headache.