Can anyone recommend the best time and method to trim a neglected (~5
years worth) Lilac tree to restore the thing back to it's main
cultivated graft?

The tree is presently about 2/3 sucker growth to 1/3 true flowering form
easy to tell at this time of year. I hope to mark the good and the bad,
but wonder if it can take such drastic pruning in a single shot.

I managed to kill one in an even worse state in a previous garden by a
combination of aggressive rejuvenation and a hard winter. I'd prefer not
to repeat the same mistake again.

Any tips for best time to do it and how much to take off per year?

Thanks,
--
Martin Brown