FWIW, pruning trees or shrubs "to size" is not a recommended practice or a generally accepted pruning method. Consistent hard pruning can weaken the tree, encouraging pest and disease issues. And to maintain the size you want, hard pruning would need to be a pretty constant practice as the lilac will want to grow to its usual size and will quickly replace/regrow what was removed.

Have you considered moving the lilac from its too confined siting and replacing it with something that stays within bounds?

Gardengal, is a lilac different from trees that are coppiced every year and often live more than a hundred?

It is different if you want flowers. Coppicing is done for wood or foliage production, not blooms.

OPs tree is a standard. Coppicing would cut it to the ground. Pollarding would create a blob on a stick with no flowers if done annually. It would also encourage long whippy growth.

I agree that it's not in a good position but if OP wishes to pollard they'll sacrifice bloom, which they know. It doesn't matter where the cuts are made as long as they're immediately above a leaf axil.

Thank you all for this good information. I hadn’t thought abouut moving it, but may have to consider that. It was planted when we built 8 years ago. I was hoping to get a couple more years in its current space. I am not familiar with the term pollarding but I will look into that. Thanks!

If you decide to move it, I will wish for you a very strong back! When I moved to my current home 3 years ago, there were three lilacs growing in deep shade in my front yard. Two were large, mature trees, and moving them was no easy task! I re-homed them to neightbors and moved the smaller one to my back yard. Then spent a couple of days recovering...

If you move the tree, you want to cut the roots on one half of the root circle in the fall (or spring) at the width of the crown. Then, in the spring (or fall), you do the other half. Then, you move it. Or wait another season to move it. Massively prune and water in well.

What's your issue with it? Rather than cutting all the branches back, you might try removing some. If you remove some branches back to the trunk each year, you'll thin it out, allowing more light into your DR. You can do this spring or fall.