Old time gardeners fed their lilacs vinegar water because they like acid soil.
I’d consider using an iron/sulpher solution.
I think keeping the yard too tidy can be a problem. I think they like dead leaves left over the roots.
What is sort of amusing to me if that Lilacs are suppose to be “easy” (compared to Tea Roses and such). Almost everyone you know had a Grammy that had a spectacular one in their yard.. filling the air with the most heavenly scent. I did read they like acidic soil (thus the one online recommendation of Epsom Salts). I now remember one woman I knew use to “mulch” hers in pine needles (thus the acid). Thanks, WWH!