DEAD-head your lilac, it will ii make you feel much better and the lilac will look better too. Few things look worse than a tatty lilac after flowering. Lilacs become rather boring when flower time is over so I make them earn their keep by acting as host for a climbing rose or a clematis. The clematis to choose would be a late-flowering one such as Clematis xjackmanii, C.
x eriostemon or a form of Clematis viticella. These can all be cut hard in early spring and the new growth will push up through the lilac in early summer. In the rose line a really vigorous one would swamp the lilac so choose one which will grow to 10 or 12 feet.
Deutzia has finished flowering also. Left alone for a few years this shrub becomes a tangle of old wood. To encourage young vigorous growth which will flower next year, cut some of the older stems completely. Deutzia requires discriminate pruning rather than a hard chop back all over.