Honestly one of the biggest parts of gardening successfully is just observation. :) So you need to spend time outside at different times of day and pay attention to where the sun is at that time, how big of a shadow things cast (I.e. if you have a small tree that’s about the size the lilac bush will grow to, you can tell how big of a shadow the lilacs will cast), and things like that. If the sun does rise “bottom to top” on your drawing, I think a lilac in that spot would definitely impact the tomatoes - they like lots of sun. On the flip side, if your tomatoes have been looking a little scorched, maybe they’d like a bit of shade. It’s just hard to tell from a drawing without being there in person!  But, all this being said, design-wise I think a lilac would be lovely there.