Q.How to prune overlong & bare to the top flowering shoots sprouting from sawn branches on a very large lilac tree?
After removing 3/4 off the old thick branches including all top growth it flowered prolifically last year. This year the shoots have grown too long & bare & there are less flowers at the very top.
Should I cut back all shoots to their base or start all over again by sawing more off the large old branches?
I’d like to keep it as a small tree as I’m doing a woodland planting underneath.
Your lilac needs rejuvenation pruning. You can do this all at once when the plant is dormant. Or, you can remove one large limb to the ground every year. Thin the excessive sprouts now present so that sunlight reaches the interior of the plant. This encourages new growth from the base, leaves low on the plant and also helps air flow which reduces the risk of powdery mildew. When a plant is top heavy and light doesn't penetrate, leaves die or fail to develop. The general rule for plants that are in leaf (not dormant) is to remove no more than 1/3 of the shrub's leaves.
Once you have finished rejuvenation, annually remove up to 1/3 of the branches, starting with the oldest ones. A shrub with young wood blooms more heavily than old, large branches.
Essentially your pruning technique was more or less a 'topping' of the tree. The upright branching that you now have is called sprouting.
I might recommend you do a hard prune on the tree; meaning your going to prune all the way down to about 12 inches from the ground. It could take a few years for the tree to come back to blooming; but you will have a much healthier and better shaped tree.