Go to your local nursery. Ask them. They will give you an entire tutorial. No one outside of your specific zone can help you as much.

It would be helpful if you stepped back and took a picture of the full yard so that any suggestions will align with the full context.

Northern California has a very broad series of growing zones…being more specific will help…I have lived in Eureka…Ukiah..San Francisco…Redding….all radically different weather zones..but still considered Northern California.

I’m in zone 10

What city?

IMO the local nursery is always the best place to ask this. Lilac in my experience do best in areas where they get a good frost . My parents had them in our landscape in Calgary Canada and they bloomed so well we had a lilac festival every year. Some plants need a real winter rest . I am an avid gardener but am really mindful of the needs of the plants first.

If you put another grass between the two it might look better than the alternating plant look. A CA native manzanita or twisted myrtle on the corners would be striking against your white house but in NoCal you have a lot of evergreen bush options.

Zone 10? I would think that’s waaay out of the comfort zone for a lilac.

Zone 10 needs special varieties of lilacs that do well with high heat and not much chilling. See https://www.descansogardens.org/gardens-and-collections/explore-the-gardens/lilacs/ Descanso is in Zone 10a.

BTW, the fragrant Descanso Gardens lilacs are blooming spectacularly well this year.

Some visual

or

Please note that the advice to seek out a local nursery does not mean a big box store. Find a privately owned garden center/nursery which might employ a trained expert. Places like that may even offer design services.